A signal processing device includes a converting unit, a filtering unit, a differential computing unit, and a phase difference computing unit. The converting unit samples two alternating signals with a predetermined period and converts the sampled level values into digital alternating signal data. The filtering unit filters the two digital alternating signal data generated by the converting unit so as to abstract digital alternating signal data having a predetermined frequency, and the filtering unit comprises an adaptive digital filter. The differential computing unit computes differentials of the digital alternating signal data generated by the filtering unit. The phase difference computing unit computes phase difference using the two digital alternating signal data generated by the filtering unit, and the two digital alternating signal data generated by the differential computing unit.