To constantly maintain a completely closed state of a cover body by forming a contact portion of a slide member and an engagement member in an inclined surface at the time when the slide member is within a locking position range and making the slide member free to slide in the unlocking direction by pressure force in accordance with closing motion from an open state of the cover body.

Inclined surfaces 21A, 31A are respectively formed on a slide member 2 and an engagement member, and these inclined surfaces 21A, 31A forms a contact portion where a claw piece 21 of the slide member 2 and a projected piece 31 of the engagement member 3 make contact with each other at the time when the slide member 2 in the opened state of the cover body 20 is within a locking position range L. That is, the inclined surfaces 21A, 31A respectively formed on the slide member 2 and the engagement member 3 are made free to slide by immoderate pressure force in accordance with closing motion from the opened state of the cover body 20, consequently, constantly maintain the cover body 20 in a complete closed state and prevent deformation and breakage of the claw piece 21 of the slide member 2.