The present invention provides a semiconductor device a comprising: a delayed-signal-generating circuit for delaying a reference pulse signal by a delay time caused by a delay component on a critical path of a target circuit by a selector included in the delayed signal generating circuit and, thereby, generating a delayed pulse signal; a detection-signal-generating circuit, having the same delay component as the selector, for generating a detection pulse signal delayed in phase by one cycle of a clock signal Ck with respect to the reference pulse signal; a delay-difference-detecting circuit for detecting a phase difference between the delayed pulse signal and the detection pulse signal; and a control circuit for adjusting the magnitude of a power-supply voltage VDD supplied to the target circuit according to the-phase difference detected by the delay-difference-detecting circuit.