The present device was conceived in order to resolve the aforementioned problem, and the objective thereof is to provide an arc chamber of a circuit breaker that enhances assemblability and ensures the consistency of the product. An arc chamber of the circuit breaker according to one embodiment of the present device comprises: a plurality of grids arranged at predetermined intervals outside a motion locus of a movable contactor and having a body portion and a leg portion extending from one side of the body portion; and a fixing cap having a receiving portion that is inserted into the leg portion in a press-fitting manner, wherein the fixing cap has a coupling portion extending to a side of the body portion at one side thereof.