Exhibit 5




December 18, 1997

HSB Group, Inc.
One State Street
P.O. Box 5024
Hartford, CT  06102-5024

Ladies and Gentlemen:

In reference to the Registration  Statement of HSB Group, Inc.  (hereinafter the
"Company")  on Form S-8 which is being filed with the  Securities  and  Exchange
Commission in connection with the  registration  of the Company's  common stock,
without par value (the "Common  Stock")  and,  where  applicable,  participation
interests (the "Participation Interests") to be offered under the Company's 1985
and 1995 Stock Option Plans and the Company's  Employees'  Thrift Incentive Plan
(the  "Plans"),  you have  asked my  opinion  as Deputy  General  Counsel of the
Company, on the validity of the issuance of the shares being registered.

In rendering this opinion,  I, or attorneys under my supervision,  have examined
the Plans, the Company's  Articles of Incorporation  and By-laws as presently in
effect, the minutes of applicable Board meetings relating to the Plans, and such
other records of the Company which I deem necessary under the circumstances.

Based on the foregoing,  I am of the opinion that the proposed issuance of up to
2,600,000  shares of Common Stock under the Company's 1985 Stock Option Plan, up
to 1,850,000  shares of Common Stock under the Company's  1995 Stock Option Plan
and up to  298,981  shares of Common  Stock,  when sold as  contemplated  by the
Registration  Statement,  will  be  duly  and  validly  issued,  fully-paid  and
non-assessable,  and that the Participation  Interests,  when issued pursuant to
the Company's Employees' Thrift Incentive Plan, will be duly and validly issued,
fully-paid and non-assessable.

I hereby consent to the inclusion in said Registration Statement of this opinion
as an exhibit thereto.

Sincerely yours,


/s/ Roberta A. O'Brien
Roberta A. O'Brien
Deputy General Counsel