An actuator for a shift by wire automatic transmission, which is applied to a shift by wire automatic transmission, wherein the shift by wire automatic transmission is configured to operate in response to an electric signal that is output in accordance with an operation state of an operation member and to perform switching to a shift range selected by the operation member, and wherein the actuator is configured to rotate a control shaft that is provided in the automatic transmission about an axis line, the actuator comprising: a case that includes an insertion hole through which the control shaft is inserted; a motor that is contained in the case and is configured to generate a driving force in response to the electric signal; a pair of gears (13, 14) that are contained in the case and are configured to transmit the driving force of the motor to the control shaft; and an output shaft that is disposed coaxially with the insertion hole between the pair of gears (13, 14) 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, wherein a part of an inner peripheral portion of the output shaft is configured with a spline hole such that the control shaft is inserted to be movable in a direction of the axis line and to be rotatable about the axis line integrally with the output shaft, and a spline portion is provided on an outer peripheral portion of the control shaft so as to engage with the spline hole, characterized by a sensor holder that is disposed coaxially with the output shaft in the case, is fitted with the control shaft to rotate integrally with the control shaft around the axis line, and holds a sensor which is contained in the case and is configured to detect the shift range of the automatic transmission, wherein the spline hole and the spline portion engage with each other with a gap in a rotation direction, the sensor holder is provided with a through-hole having a two-surface width in its inner peripheral portion, a two-surface width portion is provided as an engagement flat surface on a tip side of the control shaft so as to correspond to the two-surface width of the through hole, and an interplane distance of the two-surface width portion provided in the control shaft is slightly larger than an interplane distance of the through hole. a case that includes an insertion hole through which the control shaft is inserted; a motor that is contained in the case and is configured to generate a driving force in response to the electric signal; a pair of gears (13, 14) that are contained in the case and are configured to transmit the driving force of the motor to the control shaft; and an output shaft that is disposed coaxially with the insertion hole between the pair of gears (13, 14) 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, wherein a part of an inner peripheral portion of the output shaft is configured with a spline hole such that the control shaft is inserted to be movable in a direction of the axis line and to be rotatable about the axis line integrally with the output shaft, and a spline portion is provided on an outer peripheral portion of the control shaft so as to engage with the spline hole, characterized by a sensor holder that is disposed coaxially with the output shaft in the case, is fitted with the control shaft to rotate integrally with the control shaft around the axis line, and holds a sensor which is contained in the case and is configured to detect the shift range of the automatic transmission, wherein the spline hole and the spline portion engage with each other with a gap in a rotation direction, the sensor holder is provided with a through-hole having a two-surface width in its inner peripheral portion, a two-surface width portion is provided as an engagement flat surface on a tip side of the control shaft so as to correspond to the two-surface width of the through hole, and an interplane distance of the two-surface width portion provided in the control shaft is slightly larger than an interplane distance of the through hole.