A movable part of a circuit breaker for a gas-insulated switchgear is proposed. The movable part includes a puffer cylinder, a nozzle, and a main contact. An outer rib at the rear end of the nozzle is hooked to a hook rib at the front end of the hook part of the puffer cylinder. A chamber guide is provided inside the puffer cylinder so that the nozzle is immobilized. The main contact is mounted on the outer surface of the front end of the hook part. Thus, the number of components constituting the movable part of the circuit breaker is reduced, and the number of bolting points is reduced, thereby greatly reducing man-hours in an assembly work and preventing a main contact separation from occurring due to bolt loosening.