An electrostatic encoder (40) detects the rotation angle of a rotor (42) with great accuracy based on the change in the capacitance between electrodes arranged on a stator (41) and the rotor (42). Detection electrodes (44a to 44d) and transmission electrodes (45a to 45d) are arranged circumferentially and alternately on the stator (41). Detection signals (phase A, phase B) amplitude-modulated based on the rotation of the rotor (42) and having a mutual phase difference of 90 degrees are output from adjacent ones of the detection electrodes. Modulated signals (V1, V2) are generated by demodulating the detection signals having a mutual phase difference of 90 degrees. Applying resolver-digital (RD) conversion processing to the modulated signals allows obtaining the rotation angle of the rotor.