An external power source control system for a portable telephone includes: a
regulator
5 connected at an input thereof to a battery power source 1 or an
external power source 2 and outputting a power supply voltage; a one-shot
pulse generating circuit 6 generating a one-shot pulse when the external
power source 2 is connected to the portable telephone; a reset circuit 7
outputting a reset signal when the power supply voltage reaches a predetermined
threshold level; and a controller maintaining an output of the power supply voltage
from said regulator 5, based on the one-shot pulse, and canceling the maintenance
of the output of the power supply voltage from said regulator 5, based on
a predetermined instruction indicating that the power is not necessary.
