PURPOSE: To calculate an accurate battery charge amount by holding a voltage of the battery before a radio equipment conducts transmission/reception for the transmission/reception execution period so as to detect the voltage in an optional timing regardless of the transmission reception timing of the radio equipment.

CONSTITUTION: A voltage of a battery 1 is detected by a voltage detection circuit 2, and it includes a voltage drop in an internal resistance of the battery due to a large current caused at transmission in addition to a voltage drop through the current consumption as a whole. Furthermore, the transmission is interrupted for each prescribed time in a radio output of a time division multiplex communication equipment 6 and a comparatively large current is supplied from the battery 1, the voltage drop in response to the internal resistance is caused, resulting that an output of the circuit 2 is pulsative as shown in broken lines. A sample-and-hold circuit 3 provides an output of the circuit 2 to an A/D converter 4 when a timing signal (a) is at a low level and an output voltage of the circuit 2 is kept before the timing signal reaches a high level and fed to the A/D converter 4. Then the output of the circuit 3 is made constant from the start of transmission reception of the communication equipment 6 till its end.