A compact power supply circuit which can supply power to various apparatuses and circuits with a high degree of stability. A primary constant voltage circuit is connected to a second power supply line, which is supplied with power from a battery only when a relay is closed. The circuit supplies power to a third power supply line at a constant primary voltage. An auxiliary constant voltage circuit is connected to a first power supply line, which is always supplied with power from the battery for supplying power to the third power supply line at a constant auxiliary voltage lower than the primary voltage. A halt control circuit enables the operation of the auxiliary constant voltage circuit if power is supplied to the second power supply line and disables the operation of the auxiliary constant voltage circuit if the supply of power is interrupted for a period equal to or longer than a predetermined allowable time. The supply of power to the third power supply line is continued by the auxiliary constant voltage circuit if the temporary interruption of the supply of power to the second power supply line is within the allowable time.