An adapter, for a terminal unit, comprising a base housing having a mount surface for mounting of a terminal unit. A receptive space is defined by a plurality of walls on a part of the mount surface. A pair of guide rails are formed on the mount surface of the base housing, so as to be slidably engaged with the terminal unit. Overhanging guide plates extending from opposed side walls have edges facing to the mount surface, so as to be slidably engaged with the terminal unit. The guides rails and the edges serve to guide the terminal unit to introduce a part of the terminal unit into the receptive space. A pair of protrusions are formed on the mount surface, so as to be engaged with the terminal unit. The protrusions serve to detachably hold the terminal unit in a proper position on the mount surface to maintain the functional connection between the adapter and the terminal unit.