Provided is a drive device which can be freely attached to/detached from a lens barrel and enables the time required to detect the position of a zoom ring to be made uniform. Also provided are a control method whereby the initial process for this device is controlled by storing the results of the initial process in memory, and a method for adjusting the angular position. A series of pulses is output from an encoder (45) in accordance with the rotation of the zoom ring. A reference signal from a photointerrupter (40) is output at the intermediate position in the range of motion of the zoom ring. The current position of the zoom ring is calculated from the encoder pulses emitted after the rise or fall of the reference signal. Because the reference signal from the photointerrupter (40) is output from the intermediate position in the range of motion of the zoom ring, the time required to detect the position of the zoom ring can be made uniform.