A direct-current voltage supply circuit includes a first field effect transistor in a power line, a voltage divider including a first fixed resistor and a positive temperature coefficient thermistor between the power line and a ground line, a thyristor connected at an anode thereof to the power line, with a second field effect transistor interposed therebetween, connected at a gate thereof to a node in the voltage divider, and connected at a cathode thereof to the ground line, with a second fixed resistor interposed therebetween, and a third fixed resistor connected in parallel with the thyristor and the second field effect transistor. If abnormal heating increases the resistance of the positive temperature coefficient thermistor, the first field effect transistor is turned off and the supply of direct-current voltage is stopped.