To provide an overdischarge prevention circuit for a storage battery which uses a small number of components, has a simple circuit configuration and consumes zero current after disconnecting a storage battery from a load side for overdischarge prevention.

The overdischarge prevention circuit for a storage battery includes: a MOS relay RY1 inserted in series in a power supply line 40 from a storage battery 20 to a load 30; a normally open contact switch SW1; a normally closed contact switch SW2; and a Zener diode ZD and a resistance R1 connected in series with a photodiode 11 of the MOS relay RY1. When a terminal voltage of the storage battery 20 is equal to or higher than a predetermined value, an operation of the switch SW1 breaks down the Zener diode ZD to cause a flow of current through the photodiode 11 and maintain the MOS relay RY1 in an on state. When the terminal voltage of the storage battery 20 falls below the predetermined value, the Zener diode ZD turns off to bring the MOS relay RY1 to an off state.