PURPOSE: To eliminate back gate effect and to improve precision by using mutually independent inverters which are free of the back gate effect and have a low and a high transition voltage respectively.

CONSTITUTION: The sources of the inverter I having the high transition level and the inverter II having the low transition level are held at the same potential with a substrate. Then, when an input 100 falls from H to L, N channel transistors (TR) 6 and 8 are on and the outputs 200 and 300 of the inverters I and IIare both at L. The output 400 of an inverter 12 is at H. When the input 100 approximates to L from H, the output 200 of the inverter 200 falls to L earlier, but the output 400 is at H, so a P channel TR9 is still off, so that the output L is not transmitted to an input 11. When the input 100 further approximates to L, the output 300 of the inverter II goes up to H and the source of an N channel TR10 and the substrate are short-circuited, so the substrate side is at H and the drain side is at L; and PN junction is biased forward, so that the input 11 is apt to rise to a potential VDD-VBE. The output of the inverter 12, however, falls to L earlier than that and the P channel TR9 turns on, holding the input 11 completely at H.