To provide: a power supply detecting circuit capable of detecting power supply by a digital circuit; and a microcontroller.

This power supply detecting circuit has a flip-flop circuit (101) to store an indefinite value when power is turned on and output data of a plurality of bits, and a comparator (102) to compare the data of the plurality of bits which is output from the flip-flop circuit with a fixed value of the plurality of bits and output a power supply detecting flag in accordance with the result of the comparison.