CUNO Incorporated
             400 Research Parkway, P.O. Box 1018, Meriden, CT 06450-1018, U.S.A.
                                                       Telephone: (203) 237-5541

                                                                     Exhibit 5.1




November 17, 2004


The Board of Directors
CUNO Incorporated
400 Research Parkway
Meriden, Connecticut 06450


Re:            CUNO Incorporated Registration Statement on Form S-8

Dear Sirs:

I have acted as counsel for CUNO Incorporated, a Delaware corporation (the
"Company"), in connection with its Registration Statement on Form S-8 being
filed on the date hereof (the "Registration Statement") relating to an aggregate
of 1,200,000 shares (the "Shares") of Common Stock, $.001 par value (the "Common
Stock"), of the Company which may be issued pursuant to the CUNO Incorporated
1996 Stock Incentive Plan, as amended, or the CUNO Incorporated Non-Employee
Directors' Stock Option Plan, as amended (such plans, as so amended, the
"Plans").

In that connection, I have examined originals, or copies certified or otherwise
identified to my satisfaction, of such documents, corporate records and other
instruments as I have deemed necessary or appropriate for the purpose of
rendering this opinion, including: (a) the Certificate of Incorporation of the
Company as in effect on the date hereof; (b) the By-Laws of the Company as in
effect on the date hereof; (c) resolutions adopted by the Board of Directors of
the Company at meetings held July 25, 1996, November 17, 1998, March 23, 2000
and January 22, 2003; (d) resolutions adopted by the stockholders of the Company
at meetings held March 27, 1997, March 25, 1999 and March 6, 2003; (e) the
Registration Statement; and (f) the Plans.

Based upon the foregoing and assuming that the purchase price of the Shares
issued pursuant to the Plans will not be less than the par value of the Shares,
I am of the opinion that the Shares will, upon issuance in accordance with the
provisions of the Plans subsequent to the exercise of options or other rights
granted under the Plans, be validly issued, fully paid and nonassessable.




CUNO Incorporated                    -2-                       November 17, 2004


The opinion contained herein is intentionally and specifically limited to those
Shares of Common Stock which may hereafter be issued by the Company for delivery
pursuant to the Plans. The opinion contained herein is also limited in that no
opinion is expressed other than as to the General Corporation Law of the State
of Delaware.

I hereby consent to the filing of this opinion as Exhibit 5 to the Registration
Statement and to the reference to me under "Legality" in the Prospectuses
constituting part of the Registration Statement.

                                              Very truly yours,


                                              /s/ JOHN A. TOMICH

                                              John A. Tomich
                                              General Counsel and Secretary