<PAGE>

                                                                   Exhibit 31(b)

                         CERTIFICATION OF THOMAS J. WEBB

I, Thomas J. Webb, certify that:

      1.    I have reviewed this annual report on Form 10-K of CMS Energy
            Corporation;

      2.    Based on my knowledge, this 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 report;

      3.    Based on my knowledge, the financial statements, and other financial
            information included in this 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 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 have:

                  a) Designed such disclosure controls and procedures, or caused
            such disclosure controls and procedures to be designed under our
            supervision, 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 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 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 has materially
            affected, or is reasonably likely to materially affect, the
            registrant's internal control over financial reporting; and

      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 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.

Dated: March 10, 2005                       By        /s/ Thomas J. Webb
                                               ---------------------------------
                                                        Thomas J. Webb
                                                 Executive Vice President and
                                                    Chief Financial Officer