A mold cased circuit breaker, which includes a casing, a switching unit disposed in the casing to open or close an electric circuit, and movable and stationary contactor units present within the casing, includes an arc-extinguishing unit. The arc-extinguishing unit includes a pair of side plates facing each other with being spaced from each other, at least one first grid arranged between the side plates and spaced apart from one another with preset intervals, and a second grid coupled to upper ends of the side plates, spaced apart from the first grid, and having a bent portion with a preset angle.