A power supply unit for electronic devices has an insulating battery pack holder with a hollow portion to accommodate a battery pack detachably; a plurality of connector contacts and a switch contact formed of conductive metal and disposed inside the battery pack holder, part of each connector contact protruding toward the hollow portion, the switch contact capable of detaching from or attaching to a predetermined one of the connector contacts with a moving contact portion. When the battery pack is accommodated in the hollow portion, its electrode terminals elastically touch corresponding connector contacts to be conductively connected to a driver circuit. In removal of the battery pack from the hollow portion, when the predetermined connector contact protrudes into the hollow portion by a predetermined distance according to a reduction in urging force of the electrode terminal, the predetermined connector contact touches the switch contact to output a detection signal.