There are provided a drive device that is detachably mounted on a lens barrel making time, which is required until the position of a zoom ring is detected, uniform, a method of controlling initial processing for storing a result of initial processing in a memory, and a method of adjusting an angular position. A series of pulses are output from an encoder 45 according to the rotation of a zoom ring. A reference signal is output from a photo interrupter 40 at an intermediate position of a movable range of the zoom ring. The present position of the zoom ring is calculated from the pulses of the encoder that are generated after the rising or falling of the reference signal. Time, which is required until the position of the zoom ring is detected, is made uniform.