An actuator according to an embodiment includes a first rotating body that is rotatable around an input axis and includes a first groove part extending in a first direction, a second rotating body that is rotatable around an output axis and includes a second protruding part extending in a second direction substantially perpendicular to the first direction, a strain body that includes a first protruding part capable of being engaged with a first groove part through a gap whose direction is vertical to that of the input axis and a second groove part capable of being engaged with a second protruding part through a gap whose direction is vertical to that of the output axis and transmits a rotational torque of the input axis to the output axis, and a detection element that is attached to the strain body.