Abstract: Disclosed is a battery changing method during processing of a call without interrupting a talking path in a wireless telephone set having a supplementary battery and a main battery. In the battery changing method according to the invention, it is detected whether or not a capacity of the battery is less than a predetermined value during the call processing. If in the affirmative, it is detected whether or not a mode set key requiring a battery change is inputted by a user. If in the affirmative, electric power of the wireless telephone set is switched to the supplementary battery and a power saving mode is operated to transmit messages for maintaining the wireless link. It is then detected whether or not a key requesting a return to a normal mode is inputted during the power saving mode. If in the affirmative, electric power of the wireless telephone set is switched back to the main battery and a normal mode is resumed.