Exhibit 5.1

                       LEGALITY OPINION OF FOLEY HOAG LLP

February 8, 2005

Benjamin Franklin Bancorp
58 Main Street
P.O. Box 309
Franklin, MA 02038-0309

Ladies and Gentlemen:

      We are furnishing this opinion of counsel to Benjamin Franklin Bancorp,
Inc., a Massachusetts corporation (the "Company"), for filing as Exhibit 5.1 to
the Registration Statement on Form S-1 (the "Registration Statement") filed by
the Company with the Securities and Exchange Commission under the Securities Act
of 1933, as amended, relating to the issuance of shares of the common stock of
the Company, no par value (the "Shares") in connection with the Company's
conversion from mutual to stock form.

      In this regard, we have examined the Articles of Organization and Bylaws
of the Company, resolutions of the Board of Trustees of the Company, the Plan of
Conversion of the Company ("Plan"), and such other documents and
matters of law as we deemed appropriate for the purposes of this opinion.

      Based on the foregoing, it is our opinion that, once it has obtained all
necessary regulatory approvals and consummated the conversion pursuant to the
terms of the Plan as described in the Registration Statement, the Company will
have corporate power adequate for the issuance of the Shares in accordance with
the Registration Statement. When certificates for the Shares have been duly
executed and countersigned, and delivered against due receipt of consideration
therefor as described in the Registration Statement, the Shares will be legally
issued, fully paid and non-assessable.

      We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to the reference to our firm under "Legal Matters"
in the Prospectus which is part of such Registration Statement. This consent
shall not be deemed to be an admission that counsel is an expert within the
meaning of Section 7 of the Securities Act of 1933, as amended.


                                                  Very truly yours,

                                                  FOLEY HOAG LLP

                                                  By: /s/ Carol Hempfling Pratt
                                                      -------------------------
                                                      A Partner