EXHIBIT 5.1

                                                                         -------
                                                                          MAYER
                                                                         -------
                                                                          BROWN
                                                                         -------
                                                                          ROWE
                                                                         -------
                                                                          & MAW
                                                                         -------

April 17, 2006                                      Mayer, Brown, Rowe & Maw LLP
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SPSS Inc.
233 South Wacker Drive
Chicago, Illinois 60606

Re:   SPSS Inc. Registration Statement on Form S-3
      --------------------------------------------

Dear Ladies and Gentlemen:

We have acted as special counsel to SPSS Inc., a Delaware corporation (the
"Company"), in connection with the proposed sale of the following securities
(the "Securities") as set forth in the Form S-3 Registration Statement filed
with the Securities and Exchange Commission on the date hereof (the
"Registration Statement"): (i) shares of the Company's common stock, par value
$0.01 per share (the "Common Stock") and (ii) one or more series of debt
securities of the Company (the "Debt Securities").

Each series of the Debt Securities will be issued under an Indenture to be
entered into by the Company and LaSalle Bank National Association, as Trustee
(the "Indenture"). Certain terms of the Securities to be issued by the Company
from time to time, and the consideration to be received therefor, will be
determined by and pursuant to resolutions of the Board of Directors of the
Company as part of the corporate action taken and to be taken in connection with
the authorization of the issuance of the Securities (the "Corporate
Authorizations").

As special counsel to the Company, we have examined originals or copies
certified or otherwise identified to our satisfaction of the Company's Restated
Certificate of Incorporation, the Company's By-laws, resolutions of the
Company's Board of Directors and such Company records, certificates and other
documents and such questions of law as we considered necessary or appropriate
for the purpose of this opinion. In rendering this opinion, we have assumed the
genuineness of all signatures, the authenticity of all documents submitted to us
as originals and the conformity to authentic original documents of all documents
submitted to us as copies.

Based upon and subject to the foregoing and to the assumptions, conditions and
limitations set forth herein, we are of the opinion that:

(1) upon the completion of the Corporate Authorizations relating to the shares
of Common Stock, the due execution, countersignature and delivery of the shares
of Common Stock, and the receipt of the consideration set forth in the
prospectus contained in the Registration Statement and any prospectus supplement
relating to the Common Stock, the Common Stock will be validly issued, fully
paid and non-assessable; and




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SPSS Inc.
April 17, 2006
Page 2


(2) upon execution of the Indenture and the completion of the Corporate
Authorizations relating to a series of the Debt Securities, the due execution,
authentication, issuance and delivery of the Debt Securities of such series, and
the receipt of the consideration set forth in the prospectus contained in the
Registration Statement and any prospectus supplement relating to such series of
the Debt Securities, the Debt Securities of such series will be valid and
binding obligations of the Company entitled to the benefits of the Indenture and
enforceable against the Company in accordance with their terms, except as such
enforceability may be limited by bankruptcy, insolvency, fraudulent conveyance,
moratorium, reorganization or similar laws affecting creditors' rights generally
and subject to general principles of equity.

We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to being named in the related prospectus and any
related prospectus supplement under the caption "Legal Matters" with respect to
the matters stated therein.

                                            Very truly yours,

                                            /s/ Mayer, Brown, Rowe & Maw LLP

                                            MAYER, BROWN, ROWE & MAW LLP