PURPOSE: To evade such a case where a P-N diode is constituted in a Pch transistor TR of a transmission gate by setting the potential applied to a reference voltage input terminal or an analog input terminal at a higher level than the analog power voltage.

CONSTITUTION: The potential of the substrate 11 of each Pch TR 5a is set at a level equal to the reference voltage for plural transmission gates 1a-1c which select the comparison voltage 6b and transmission gates 1d and 1e which input the analog input (ANIN) to be converted and the selected voltage 6b to a comparator 6. Meanwhile a level shift circuit 8 converts the control signal set at an H level at the Pch side into the level of the reference voltage form the level of an analog power supply so that the reference voltage, i.e., the substrate potential of the TR 5a of each of gates 1a-1e is set at the H level of the control signal which is applied to a gate electrode 4.