According to one embodiment, a power supply voltage monitor circuit includes a constant voltage circuit, a level shift circuit, a clamping circuit, a first differential circuit, and a second differential circuit. The first differential circuit include a differential unit receiving a constant current supplied from a current source and outputting an output voltage in accordance with a potential difference between a first input voltage obtained by subjecting the second constant voltage to resistive division and a second input voltage obtained by subjecting the power supply voltage to resistive division, and an output unit outputting a rectangular signal in accordance with the output voltage of the differential unit. The second differential circuit turns off the current source of the differential unit depending on a potential difference between the first constant voltage and the first input voltage to thereby control the output operation of the output unit, in a case where the clamping circuit does not fix the first constant voltage. On the other hand, The second differential circuit turns on the current source of the differential unit depending on a potential difference between the clamping voltage and the first input voltage.