To supply a non-volatile memory data protecting device in which even when a main power source is turned off, the main power source is switched to a backup power source only when data are written in a non-volatile memory and the backup power source is switched to the main power source again when the backup is ended.

This device is provided with a power source part 2 for supplying a power to the non-volatile memory 1 and a backup power source 3 (secondary battery such as a lithium ion battery), and the power source is switched by a switch 4. The switching of the switch 4 is controlled by a control part 5 for controlling the switching of the supply power source. The operation of the control part 5 is decided according to the results of a voltage decrease detecting means 6 for detecting the decrease of the power supply voltage of the power source 2 and a writing detecting means 7 for detecting that data are being written in the non-volatile memory 1.