To realize a class D amplifier whose output is isolated from a voltage source and in which a maximum output is properly set.

The amplifier is mainly made up of a primary circuit having a transformer 21 and switch means 11, 12 of half bridge, full bridge or push-pull configuration and a secondary circuit having the transformer 21 and combination of one or two switch means 23, 24 in pairs driven complementarily. Then a 3rd signal for driving the secondary side obtained by an output of a logic circuit 19 receiving a 1st signal for driving the primary side and a 2nd signal subject to 2-state modulation of an amplified signal is used to drive the secondary circuit. Thus, the 2-state modulation signal is amplified for the power and the resulting signal is given to a low-pass filter 10 to realize desired isolation and power amplification.