To provide a cam mechanism for controlling a transmission that can control a position of a control rod on the transmission side to a position matching to a present speed of a bicycle as accurately as possible.

This cam mechanism for controlling a transmission controls the transmission in which a change gear ratio changes according to moving positions of a plurality of places assigned to a control rod. The cam mechanism comprises a cam portion 20 and a driven portion 80. The cam portion 20 comprises a pair of first and second cam surfaces 30, 40 and a groove portion 50 between the cam surfaces 30, 40. The driven portion 80 is deposited in the groove portion 50. The control rod moves to the moving positions of the plurality of places interlocking with displacement of the driven portion 80.