To solve the problem in which in conventional drive units, a rocking member such as a reset lever is disposed outside a rotary member such as a cam gear to reduce utilization efficiency of a space inside the drive unit.

A transmission mechanism comprises: a rotary member that includes an input part rotating around a rotary shaft according to driving of a motor, having a thickness in a direction of the rotary shaft and receiving driving force of the motor, and an output part having a thickness in the direction of the rotary shaft and outputting the driving force to an external first drive member; and a rocking member that includes an engagement member engaged with the rotary member so as to rock relatively to the rotary member around a rocking shaft in parallel with the rotary shaft and outputs the driving force to an external second drive member. A thickness of the engagement member and a thickness of at least one of the input part and the output part are at least partially overlapped with each other in the direction of the rotary shaft.