PURPOSE: To prevent the restarting of a power supply circuit when a thermal guide is once operated and becomes the open state and thereafter a make state is obtained again.

CONSTITUTION: When an AC starting voltage (Vcc) 20 is inputted, a current flows through a first capacitor 5, and the capacitor 5 is charged. The gate of a thyristor 13 is triggered, a photocoupler 12 for starting a power supply is operated and a power supply circuit is started. When a thermal guide 9 is operated by heat and becomes the open state while the power supply is operating, the holding current for the thyristor 13 is stopped. Therefore, the photocoupler 12 is also stopped, and the power supply circuit is stopped. Even if the thermal guide 9 becomes the make state by cooling of external air under this state, the capacitor 5 is in the fully charged state, and the thyristor 13 is not triggered. When the make state is obtained again after the thermal guide 9 is once operated and becomes the open state, the restarting of the power supply can be prevented.