PURPOSE: To make the mounting of a ready-made built-in type wiring instrument to a mounting frame used for mounting to a wall or the like in freely attachable and detachable, to obtain a nice appearance and to facilitate execution.

CONSTITUTION: A printed circuit board 26 mounted with the parts of a circuit and formed with the circuit is housed in the inside of a body 31. The central part of a cover 22 in the breadthwise direction is swelled so as to approximately correspond to the shape of a mounting frame 40. An optical wireless receiver X is mounted to the wall by fixing it with a screw to a box built in the wall or the like through a mounting hole 43 of the mounting frame 40 and mounting a decorative plate 44 equipped with a hanging lock pawl 45 at its bottom side to the mounting frame 40 by means of a rectangular shape formed with a light transparent part 47 corresponding to the light transparent window 35 of the cover 22. The case 20 of the light wireless receiver X is formed to the dimension of the module of a standardized ready-made wiring instrument. Thus, the nice appearance can be obtained and the execution can be facilitated.