A switching amplifier including a PWM circuit 14, a driver 15, a switching output circuit 16 and an LPF 17 is provided with a fluctuation detection circuit 12 for detecting fluctuations in the power supply voltage. The fluctuation detection circuit 12 includes detection resistors 122 and 123 connected in series between a positive power supply 19 and a negative power supply 20. A fluctuating signal RPL occurring at a fluctuation detection terminal 121 is supplied to the inverted input terminal of a differential amplifier 131 of a non-inverted amplifier circuit 13. The non-inverted amplifier circuit 13 inverts the fluctuating signal RPL, mixes the inverted fluctuating signal into the input signal e.sub.S, amplifies the mixed signal, and supplying the amplified signal to the PWM circuit 14.