An ID code is wirelessly sent to a device, such as a keyless-entry system or a keyless-starter system mounted on an automobile, from a portable transmitter. Electronic components including an electronic circuit board, a transponder and a battery are all contained in a resin case. A mechanical key that is to be used to open the door when the battery voltage is too low to operate the system is also contained in the case. The battery contained in the case is covered with the battery cover which is fixed to the case with screws. The battery cover is further covered with a sliding cover that slides in the same direction in which the mechanical key is taken out. The sliding cover and the mechanical key is locked in their respective positions by hooks which are released with a common lever.