A power supply circuit that withstands voltages greater than or equal to a voltage capacity and prevents an increase in circuit area and manufacturing costs. The power supply circuit includes a first transistor for receiving a DC voltage and generating an internal power supply voltage. A clamp circuit is connected to the first transistor. The clamp circuit is activated when the DC current voltage is an excessive voltage to clamp the internal power supply voltage at a predetermined voltage that is less than the excessive voltage. A gate voltage control circuit is connected to the first transistor and the clamp circuit to supply the gate of the transistor with a control voltage so that the internal power supply voltage decreases when the clamp circuit is activated.