<PAGE>



                                  EXHIBIT 31.1

                      Chief Executive Officer Certification
                       Pursuant to 18 U.S.C. Section 1350,
      as adopted pursuant to Section 302 of the Sarbanes-Oxley Act of 2002

I, John H. Castle, certify that:

    1.  I have reviewed this quarterly report on Form 10-Q of Southern Michigan
        Bancorp, Inc.;

    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(s) 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)) 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) 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

         c) 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(s) and I have disclosed, based
        on our most recent evaluation of internal control over financial
        reporting, to the



                                      E-2

<PAGE>



        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: November 13, 2003                 /s/ John H. Castle
      -----------------                 ----------------------------------------
                                        John H. Castle, Chief Executive Officer


                                      E-3