A rotation-detecting apparatus includes the following: a rotor coil provided on a rotor; detection coils provided on stators; a control circuit that detects the relative rotational angle between the rotor and the stators by processing detection signals induced in the detection coils as a result of the rotor coil being excited by an excitation signal; and a communicating means for performing data communication with an external device. The control circuit in the rotation-detecting apparatus has functionality whereby a switch signal that turns on and off at preset rotational angles is outputted on the basis of the aforementioned detection signals. Also, the control circuit is designed such that the set rotational angles for the aforementioned switch signal can be changed by the external device via the abovementioned communicating means.