A rotational angle detecting device continues a portion of operation by electric power from a battery when an electric switch is off. The rotational angle detecting device includes a sensor, a first calculator, a second calculator and a communication portion. The sensor detects a detection value that is variable according to a rotation of a detection object. The first calculator starts calculating first rotational information associated with the rotation of the detection object based on the detection value when the electric switch is turned on. The first calculator stops calculating the first rotational information when the electric switch is turned off. The second calculator calculates second rotational information associated with the rotation of the detection object regardless of an on/off state of the electric switch, based on the detection value. The communication portion outputs the first rotational information and the second rotational information to a controller.