a darlington circuit and a resistor r11 also the current compensating circuit 15 comprises a variable resistor r12 which is connected to the voltage source vcc in this device the input bias currents of the differential amplifier 11 and the buffer amplifier 14 are compensated by a bias compensating current ib 8 which is supplied from the voltage source vcc through the resistor r12 thus the loss of charges in the capacitor 13 becomes less and it is possible to decrease the error of peak level detection caused by change of the mark space ratio fig5 shows another embodiment which shows the current compensating circuit 15 shown in fig1 but without the transistor q7 in this case a bias compensating current ib 9 of the current compensating circuit 15 is obtained from the collector of the transistor q8 directly and is supplied to the differential amplifier 11 and the buffer amplifier 14 to compensate for the input bias currents also the buffer amplifier 14 shown in fig4 may be replaced with the same constitution as the amplifier 14 shown in fig1 which has transistors q5 and q6 and a resistor r6 as shown in fig6 moreover the variable resistor r12 shown in fig4 may be connected to other.