A lid-fastening structure for fastening a lid using a bolt, which is rotatably held in the lid for closing an adjustment opening. An insertion hole for inserting the bolt is formed in the lid. A portion of the circumferential edge of the insertion hole is cut out in a way that allows the bolt to slide, and a backslide-limiting opening for limiting the return of the bolt to the insertion hole is formed so as to be continuous with the circumferential edge. A supporting/fastening hole is formed continuous with the backslide-limiting opening to a smaller radius than is the insertion hole, the supporting/fastening hole being cut out so as to allow the bolt to advance.