a signal level 46 refers to a level control circuit which includes operational amplifier 22 being an inverting amplifier resistors 28 and 30 for setting a gain of the operational amplifier 22 a resistor 29 for biassing a capacitor 31 for stabilizing the operational amplifier 22 which has an amplification limited in high frequency to keep it from oscillating switch means 24 consisting of high speed semiconductor analog switches 24a and 24b respectively connected with the output terminal 20 via a resistor 37 and the output of the operation amplifier 22 via a resistor 38 wherein the resistor 37 has an impedance to keep the signal of the output terminal 20 from being influenced in a high frequency region and to transmit a low frequency region an operational amplifier 23 having high impedance fet inputs a large open loop gain and a small drift of an offset voltage a capacitor 34 for stabilizing the operation of the operational amplifier 23 which has an amplification limited in high frequency to keep it from oscillating capacitors 35 and 36 respectively for holding signals sampled by the switch means 24 and being connected with the inputs of the operational amplifier 23 and a resistor 32 for connecting an output of the operational amplifier 23 with an input of the operational amplifier 22 via an output terminal 33 27 indicates an output terminal of the level compensation.