To provide a cover device comprising a pair of cover members having such structure that fitting surfaces of the cover device are securely tight on each other.

This cover device comprises a first cover 1 having an opening part, and a second cover 2 to close the opening part. The first cover 1 and the second cover 2 have an outer circumferential surface 1c and an inner circumferential surface 2c to get engaged with each other to have rattling. A screw member 3 is inserted into a boss part 2e in the second cover 2 to be driven into a boss part 1e in the first cover 1, so that the first and the second covers are coupled with each other. End parts of the boss parts 1e and 2e are formed diagonal, and by fastening the screw member 3, the boss parts 1e and 2e relatively make micro-movement to apply the outer circumferential surface 1c and the inner circumferential surface 2c close contact to each other. In a coupled state, therefore, the second cover 2 is prevented from moving sideways, but favorable drip-proof and light-shielded state of the engagement parts can be obtained.