PURPOSE: To provide a charging preventing circuit of simple constitution which prevents improper charging of a battery with a leak current of a reverse blocking diode for blocking a reverse current from a main power supply to the battery of a backup circuit, in a backup circuit for driving a semiconductor memory with the battery in place of the main power supply when it is interrupted.

CONSTITUTION: A backup battery 10 and a reverse current blocking diode D0 for preventing a current from flowing in the reverse direction from a main power supply to the battery are connected in series, and a diode D1 of the same type as that of the reverse blocking diode D0 in parallel to the battery 10.