PURPOSE: To obtain a small peak voltage detection circuit with a large time constant using an IC by controlling constant at discharge according to a DC current amplification factor of a transistor.

CONSTITUTION: When the voltage of an input terminal 10 exceeds the voltage of the output terminal 11, current flows through to a current mirror circuit made up of transistors TRQ2 and TRQ5, Q6 and Q9, a load resistance and the like whose saturation is prevented with a transistor (TR)Q1 and a clamping transistor TRQ11 according to the peak voltage. Then, with the on-control of a transistor TRQ3, a capacitor C is charged according to the peak voltage. Subsequently, when the voltage of the terminal 10 drops, the transistor TRQ3 is turned OFF and the capacitor C is discharged with a large time constant corresponding to the product of the resistance value of a resistance R5, a DC amplification factor of a transistor TRQ4 connected in series thereto and the capacitance value of the capacitor C to lower the output voltage of the terminal 11. This provides a small peak voltage detection circuit with a large time constant using an IC.