To prevent useless error tone ringing in power interruption while maintaining a quick call function and a function for notifying radio disconnection of a master unit in telephone communication by an error tone.

A slave unit detects a charge state of a battery which is its power supply, and when charge disconnection is detected, judges that is hooked off and performs line capturing with the master unit. In addition, the slave unit has a function for ringing the error tone when communication with the master unit is disabled in the line capturing and outside line connection. In such a cordless telephone system, when the charge of the battery is disconnected, it is judged whether or not a synchronizing signal from the master unit is disconnected, when the synchronizing signal is disconnected, it is judged that the whole power supply falls by the power interruption, etc., and the error tone is not rung even the communication with the master unit is disabled.

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