PURPOSE: To eliminate the influence of noise, etc. by the method wherein, when the power supply voltage is below a specified value, a voltage detector circuit which prohibits the operation of a circuit connected to the power supply is formed so that the detector output has hysteresis characteristic.

CONSTITUTION: TTL circuit 42 outputs a level signal opposite to the signal level to the signal level to be impressed to input IN when input OE is at a high level, and makes its output a high impedance regardless of the level of IN when input OE is at a low level. Voltage detector circuit 41 connected here constrols multi-emitter type transistor Q43 as variation in the voltage at node N has hysteresis characteristic with respect to the variation in power supply voltage VCC. Consequently, after VCC rises and reaches a specified value, even if VCC drops due to the influence of noise, etc., TTL circuit 42 outputs a correct signal corresponding to the input signal, so that a stable operation can be performed.