A rotational angle detecting device has a detecting element that detects a rotating magnetic field of a magnet that is variable according to a rotation of a motor. A rotational angle calculator calculates a rotational angle and a rotation number. A power supply failure determining circuit has a volatile memory that stores power supply failure information indicating that power supply failure occurs in which electric power is not supplied to the rotational angle detecting device from a battery. A communication portion outputs the rotational information and output information corresponding to the power supply failure information to the controller. The communication portion receives a notifying signal after the controller receives the output information indicating that the power supply failure occurs. The volatile memory stores the power supply failure information indicating that the power supply failure occurs after the power supply failure occurs until the communication portion receives the notifying signal.