A comparator circuit includes a current mirror circuit, a differential pair, and a first current source between first and second power supply lines. The differential pair includes a first MOS transistor of enhancement mode n-type having a gate electrode at which an input signal is supplied, and a second MOS transistor of depletion mode n-type. A source of the second MOS transistor is connected with a source of the first MOS transistor and a threshold voltage of the second MOS transistor is lower than that of the first MOS transistor. A gate electrode of the first MOS transistor is formed by polycrystalline silicon which contains a p-type impurity. A gate electrode of the second MOS transistor is formed by polycrystalline silicon which contains an n-type impurity and is connected with the first power supply line. An output signal is output based on a drain voltage of the second MOS transistor.