PURPOSE: To enable the setting of many conditions by a small number of input terminals by providing a decoder applying a control potential to a middle point of a resistor train connected in series between a power source and a ground, connecting a pair of buffers to which the potential is supplied, and generating a control signal in response to its output.

CONSTITUTION: A resistance train 10 is connected in series between a power source and a ground, and between a resistor 10a and a resistor 10b of the resistance train 10, a control input for setting an operating condition of a control circuit 3 is given. Two buffers 11a, 11b have an intermediate potential of a ground potential and a power supply potential, and a potential VA between the resistors 10a, 11c and a potential VB between the resistors 10b, 10d are inputted thereto. A decoder 12 receives outputs of the buffers 11a, 11b, generates control signals C1-C3, and supplies them to the control circuit 13. That is, with respect to two kinds of control potentials of a power source level or a ground level, the respective control signals are obtained, and also, while an input is set to a floating state, a control signal for showing a third state is obtained, and from one input terminal, three kinds of control signals are obtained.