In one embodiment, to maintain the operation stability of a semiconductor device even when an external voltage changes. An input signal discrimination unit operates with a power supply potential supplied from a first power supply line VDDI. The input signal discrimination unit compares an input signal VIN with a reference potential Vref. The comparison result is inverted into a signal V0 by an inverter INV1. A power supply sensor circuit monitors the potential of the first power supply line VDDI. If an external potential VDDI falls below a reference potential VX, the power supply sensor circuit turns on a second current source. When the second current source is turned on, an operating current is supplied to a discrimination unit from the second current source as well as a first current source.