The aim of the present disclosure is to provide a watchdog timer that can perform a fault diagnosis during the actual use of a semiconductor device. In a semiconductor device provided with a watchdog timer, the watchdog timer includes a counter; a counter control circuit that changes a count value of the counter to a desired value in the refresh period of the count value; and a fault diagnosis module. The fault diagnosis module includes a suppressing circuit that suppresses generation of a reset signal to the exterior of the watchdog timer in the refresh period; and a holding circuit that holds the reset signal.