PURPOSE: To enable the common use with a display means, by displaying the voltage drop of cells through flickering of frequency display.

CONSTITUTION: It is detected that a terminal voltage of a memory backup power supply is a specified voltage or less, so that the display means such as frequency can be flickered with this detection signal. For example, to a power supply 1, a voltage drop detection circuit 2, Schmitt trigger circuit 3, and astable multivibrator 4 are connected in series, and a pulse signal is outputted if the voltage drops. On the other hand, the power supply 1 is connected to a control and storage circuit 6 via a diode 5. The pulse signal and the control signal are applied to a frequency indicator 10 via a buffer circuit 8 with a segment recorder 7. Further, a scanning signal is applied from the storage circuit 6 via a buffer circuit 9. Thus, the frequency indicator 10 cause flickering.