An actuator is applied to a shift by wire automatic transmission that operates in response to an electric signal output in accordance with an operation state of an operation member and that performs switching to a selected shift range, and rotates a control shaft provided in the automatic transmission about an axis line. The actuator includes: a case including an insertion hole through which the control shaft is inserted; a motor generating a driving force; gears contained in the case and transmitting the driving force to the control shaft; and an output shaft disposed coaxially with the insertion hole between the gears and the control shaft in the case, the output shaft allowing the control shaft to be inserted through the output shaft such that the control shaft is movable in a direction of the axis line and is rotatable about the axis line integrally with the output shaft.