PURPOSE: To enable a cover to be opened so as to expose required parts inside a case by a method wherein the cover is mounted on the case in a vertically slidable manner and fixed at a prescribed sliding position.

CONSTITUTION: A cover 16 is provided covering the front side of a case 11 to protect a printed board 14 and a wiring connection terminal 15 located at the front of the case 11. The cover 16 is lifted upward sliding on slide ribs 12 at both the ends of the case 11, whereby the front side of the case 11 is gradually opened, and a protrudent engaging part 13 and a protrudent engaged part 18 are engaged with each other to fix the cover 16 at a prescribed sliding position, and the front side of the case 11 is partially exposed. That is, the prescribed position of the engaged part 18 formed on a slide groove 17 of the cover 16 is so set as to enable the fixing of the cover 16 by the engagement of the engaged part 18 with the engaging part 13 of the case 11 to be done at a point where a wiring connection terminal 15 located at the front of the case 11 is exposed.