To eliminate the need for or reduce mechanical adjustment of a detecting element regardless of external size of an encoder.

There are provided a first amplifier 13 for amplifying A-phase signal outputted from a first position detecting means, an adder 14 for adding a B-phase signal to the A-phase amplified signal outputted from the first amplifier, a phase measuring means for adjusting a gain of the first amplifier so that a phase difference between the output signal of the adder 14 and the A- phase signal is 90° while observing the output signal of the adder 14 and the A-phase signal, and a second amplifier 15 wherein, being provided to an output stage of the other 14, a gain is so adjusted that the amplitudes of both signals are identical while observing the output signal of the adder 14 and the A-phase signal.