An expanding apparatus includes a housing with a bearing surface on which a rear portion of a computer is to be placed, and engaging projections arranged on the bearing surface so as to be fitted individually in engagement holes of the computer. As an operating lever provided on the housing is swung up or down, the engaging projections is moved between a first position in which the computer is connected to a first expansion connector of the apparatus and a second position in which the connection is removed. The apparatus is provided with a photointerrupter and an actuating piece which are arranged in the housing, while the operating lever has a push projection which causes the actuating piece to switch the photointerrupter on and off as the lever is swung. When the operating lever is swung from a home position toward a release position with the computer connected to the external expanding apparatus, the photointerrupter detects this and delivers a warning signal to the computer.