<PAGE>




                                                                   EXHIBIT 31(b)


                           DUSA PHARMACEUTICALS, INC.

                                  CERTIFICATION

I, Richard C. Christopher, certify that:

1. I have reviewed this quarterly report on Form 10-Q of DUSA Pharmaceuticals,
   Inc.;

2. Based on my knowledge, this quarterly report does not contain any untrue
   statement of a material fact or omit to state a material fact necessary to
   make the statements made, in light of the circumstances under which such
   statements were made, not misleading with respect to the period covered by
   this quarterly report;

3. Based on my knowledge, the financial statements, and other financial
   information included in this quarterly report, fairly present in all
   material respects the financial condition, results of operations and cash
   flows of the registrant as of, and for, the periods presented in this
   quarterly report;

4. The registrant's other certifying officer and I are responsible for
   establishing and maintaining disclosure controls and procedures (as defined
   in Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal control over
   financial reporting (as defined in Exchange Act Rules 13a-15(f) and
   15d-15(f)) for the registrant and we have:

     a) Designed such disclosure controls and procedures to ensure that material
        information relating to the registrant, including its consolidated
        subsidiaries, is made known to us by others within those entities,
        particularly during the period in which this quarterly report is being
        prepared;

     b) Designed such internal control over financial reporting, or caused such
        internal control over financial reporting to be designed under our
        supervision, to provide reasonable assurance regarding the reliability
        of financial reporting and the preparation of financial statements for
        external purposes in accordance with generally accepted accounting
        principles;

     c) Evaluated the effectiveness of the registrant's disclosure controls and
        procedures and presented in this report our conclusions about the
        effectiveness of the disclosure controls and procedures, as of the end
        of the period covered by this quarterly report based on such
        evaluation; and

     d) Disclosed in this report any change in the registrant's internal control
        over financial reporting that occurred during the registrant's most
        recent fiscal quarter (the registrant's fourth fiscal quarter in the
        case of an annual report) that materially affected, or is reasonably
        likely to materially affect, the registrant's internal control over
        financial reporting; and

<PAGE>




5. The registrant's other certifying officer and I have disclosed, based on our
   most recent evaluation of internal control over financial reporting, to the
   registrant's auditors and the audit committee of the registrant's board of
   directors (or persons performing the equivalent functions):

     a) All significant deficiencies and material weaknesses in the design or
        operation of internal control over financial reporting which are
        reasonably likely to adversely affect the registrant's ability to
        record, process, summarize and report financial information; and

     b) Any fraud, whether or not material, that involves management or other
        employees who have a significant role in the registrant's internal
        control over financial reporting.


Date:  May 9, 2005                   /s/ Richard C. Christopher
                                     --------------------------
                                     Richard C. Christopher
                                     Vice President, Finance and Chief
                                     Financial Officer (principal financial
                                     officer)