To provide a locking structure not only reliably locking but also easy in operation in a dial lock type cover member provided at an opening of a casing.

The locking structure of a cover member includes: openings 5, 6 provided in the casings 3, 4; the cover member 7 covering the openings 5, 6; a dial 8 for operating the cover member 7 to open and close by rotating operation; a slide member 9 engageable with the cover member 7 and locking the cover member 7 in the state of covering the openings 5, 6; and a locking releasing mechanism for rotating the dial 8 at a predetermined angle or more to engage the dial 8 with the slide member 9, and sliding the slide member 9 to thereby release the locking state where the cover member 7 covers the openings 5, 6. The locking releasing mechanism includes: an engagement claw 81 provided on the dial 8; an engagement pin 92 provided on the slide member 9; and a spring member 15 for biasing the dial 8 in the return direction.