A signal conversion device includes a first signal input terminal, a second signal input terminal, and a detector. A first AC signal is supplied to the first signal input terminal from a signal source. The second signal input terminal is supplied with an in-phase signal that is in phase with the first AC signal and an opposite-phase signal that is opposite in phase with the first AC signal. The second signal input terminal is supplied with a second AC signal whose phase has been switched by a switchover between the in-phase signal and the opposite-phase signal from the signal source. The detector detects, using the supplied first AC signal, whether the supplied second AC signal is the in-phase signal or the opposite-phase signal.