PURPOSE: To hold a voltage of an output terminal constant by so disposing conductive type transistors of a power source side and a ground side that threshold value changes of the transistors are symmetrical to an output terminal as the center.

CONSTITUTION: An absolute value of a threshold value of an NMOS transistor is assigned to be Vth, and an absolute value of a threshold value of a PMOS transistor is assigned to be Vtp. Resistors 31, 32 are so designed that a voltage of a between a NMOS 24 and a PMOS 18 becomes voltage Vdd/2 of a half of a power source voltage Vdd. In this case, a drain voltage of the NMOS 24 becomes (Vdd/2+Vtn), and a drain voltage of the PMOS 18 becomes (Vdd/2-Vtp). Since a gate voltage of an NMOS 25 becomes (Vdd/2+Vtn) and a gate voltage of a PMOS 19 becomes (Vdd/2-Vtp), a voltage of an output terminal 1 becomes Vdd/2. When a load is connected to the terminal 1 and hence the voltage of the terminal 1 is raised beyond Vdd/2, the NMOS 25 is turned off, and when it is lowered beyond the Vdd/2, the PMOS 19 is turned off, and the terminal 1 is held at the Vdd/2.