CIRCUIT BREAKER

PROBLEM TO BE SOLVED: To provide a compact circuit breaker having superior reliability at low cost with reduced number of part items and man-hours. SOLUTION: This circuit breaker is provided with a main circuit constituted of a power source side terminal board 11, facing opposite contacts 12, 13, a movable contact board 14, an over-current relay and a load side terminal board 15, an opening, closing and tripping mechanism 18 for opening, closing and tripping a circuit contact by rotating the movable contact board 14, an operation handle for opening, closing and tripping operation from the outside, a tripping device 19 for operating the opening, closing and tripping mechanism 18, an arc extinguishing unit 22, a main body case 23 for integrally housing the parts, and a main body cover 24. In this case, an arc extinguishing unit 22 positioning structure, such as a combination of a projection 26, a recessed part 27, a groove and a rib, is provided in the arc extinguishing unit 22 and the main body case 23.