An adapter is provided with a connecter portion configured to be detachably attached to the main body of the electric power tool and at least one battery receiving portion configured to be detachably attached to the battery pack. The battery pack attached to the battery receiving portion is electrically coupled with the main body of the power tool attached to the connecter portion via a power supply circuit of the adapter. The adapter is provided with a measuring portion configured to measure an index corresponding to a charged level of the battery pack and a signal receiving portion configured to receive an alarm signal outputted from the battery pack, and is configured to stop or restrict power supply to the main body of the electric power tool based upon a measurement by the measuring portion and the alarm signal that is received.