A power supply unit for extending a function of an apparatus (image pickup apparatus) unloadably loaded with a flat parallelopiped battery having a roughly square principal surface includes the flat battery, and a power supply (for example, a control grip including a cell or cells and a control portion which includes a power supply button, a shutter button, a zoom switch) electrically connected to the flat battery through a plate having the same shape as that of the flat battery and detachably mounted, projected portions are formed at both ends in the longitudinal direction of a back surface opposite to an insertion surface of the plate, along the back surface, and one of the projected portions is formed with a locked surface to be locked by a lock means (lock pawl) possessed by the apparatus, whereby mis-insertion of the plate into the apparatus is prevented, and a function of the apparatus is extended.