A switching power supply has first and second switching units and a control unit. Each switching unit has a switching element performing a switching operation to intermittently receive electric current from a battery while accumulating electric power in a coil and discharging this power to an output terminal. A resistor earthed is serially connected with each switching element. The resistance of the first resistor receiving the first current from the first switching element is lower than that of the second resistor receiving the second current from the second switching element. A maximum value of the first current is higher than that of the second current due to the difference between the resistors. The control unit controls the switching operations of the elements to boost the voltage of the battery and stops the switching operations in response to the first current exceeding an upper value.