In a battery loading and unloading mechanism, a battery formed in a flat rectangular parallelepiped and having an almost square main surface is loaded and unloaded to and from a device. The battery is formed with projecting portions at longitudinal both ends of a back surface thereof and along the back surface, whereby the battery can be prevented from being erroneously inserted into the device. The projecting portion has an inclined surface acutely inclined with respect to the back surface, and when the battery is unloaded from the device, unloading of the battery from the device is accelerated by the inclined surface undergoing contact pressure of the retaining means. The device is formed with a notched portion which is adapted to expose the other projecting portion of the battery when the battery is loaded, thereby facilitating unloading of the battery.