To provide a new battery unit that enables a small, low-cost, and power-saving body device to be formed in an eco-friendly manner, and to provide a power supply device which has the battery unit.

A power supply battery 6 is boosted by a boosting processing circuit 13 so as to form a DC power supply on the battery unit side. An AC adaptor connection detecting circuit 12 monitors whether an AC adaptor 3 is connected to a body device 4B from a voltage of an AC adaptor terminal 4a of the body device 4B. When the wireless power supply of the body device 4B is in use or when the power supply from the AC adaptor 3 is stopped for some reason, the output voltage of the boosting processing circuit 13 is boosted from an idle voltage to a prescribed voltage by a boosting control circuit 14, on the basis of a voltage switching command of the AC adaptor connection detecting circuit 12 so as to supply power from the cathode side of a reverse-current preventing diode 11 to the body device 4B, thereby eliminating the need for providing a DC-DC converter in the body device 4B.