PURPOSE: To provide a rotation angle detecting device which is reduced in a cost and high in reliability by a method wherein the one of repeating patterns is specified.

CONSTITUTION: The waveform of a detecting signal (a) detected by an electromagnetic pickup 212 is a waveform in which only a part facing a tooth A is high in voltage amplitude. In which case, by using a zero cross detecting circuit 22 employing a charging and discharging circuit and a diode lamp, zero cross detection is effected on the waveform of the output signal (a) in order to detect only an engine reference angle. In which case, a detecting waveform is fed as the waveform of an output signal (b). There is a need to separate the waveform of the output signal (a) by means of a voltage level in order to detect a cylinder reference position. In which case, a maximum voltage is held by a peal hold circuit 23 employing an OP amplifier, a diode, and a capacitor. By detecting whether the waveform of the output signal (a) is higher or lower than a reference voltage based on approximate 70% of the holding voltage by means of a detecting circuit 24, only a cylinder reference position can be detected.