To enable electric power to be supplied only when a power source adapter is properly connected to a device.

This power source adapter connecting detecting system for detecting a suitable connection of a power source adapter to a device comprises a connection determining means 7 provided in the power source adapter for generating a connection determining signal to output it to the device, and connection determiners 2, 3 provided in the device supplying electric power to the device from the power source adapter after the detection of the suitable connection to the power source adapter, based on the connection determining signal immediately after the connection to the power source adapter.