PURPOSE: To protect external machine block with high safety by applying power supply voltage on the external machine block, comparing the applying voltage of each battery block with a reference voltage thereby detecting low voltage state of power supply block reliably with high safety.

CONSTITUTION: Supply voltage from a power supply block 1A is applied or interrupted onto/from an electronic machine block 7A by turning a switch block S1 ON, OFF. In a low voltage detecting circuit 2A, a voltage comparing means 3 is applied with voltages from respective battery blocks 1a-1c while furthermore provided with a reference voltage from a reference voltage block 4A. The voltage comparing means 3 compares the applying voltages from respective battery blocks 1a-1c with the reference voltage and transmits a detection signal P1 representing the low voltage state of the battery blocks 1a-1c when the applying voltage drops below the reference voltage.