A molded case circuit breaker comprising: an upper external box defining a rear space; a lower external box coupled with the upper external box and defining a front space divided from the rear space together with the upper external box; a fixed contact unit provided on one side of the front space and electrically connected to one of a power supply and a load; a movable contact unit installed in the front space (S 1) to be movable and being in contact with the fixed contact unit or being separated from the fixed contact unit; a switching device installed in the rear space (S 2) and operating to allow the movable contact unit to be in contact with the fixed contact unit or to be separated from the fixed contact unit; an operation device installed in the front space (S 1) and the rear space (S 2) and transferring the movable contact unit according to operation of the switching device; and an electrode shaft installed on one side of the upper external box, corresponding to an outside of the front space, wherein the operation device comprises: a first link element comprising an upper link connected to the switching device and a lower link connected to the upper link; a second link element installed to be pivotable around the electrode shaft, one side of the second link element being connected to another side of the lower link to be pivotable; and a third link element with one side connected to another side of the second link element to be pivotable and another side connected to the movable contact unit to be pivotable, characterized in that the upper external box is provided with a shaft mounting groove formed by denting a part of a top surface of the upper external box downwards to allow the electrode shaft to be mounted thereon, wherein the upper external box is provided with a link penetration slot formed by cutting a part of the shaft mounting groove to allow the second link element fixed to the electrode shaft to penetrate thereinto, and wherein the second link element is formed together with the electrode shaft as a single body. an upper external box defining a rear space; a lower external box coupled with the upper external box and defining a front space divided from the rear space together with the upper external box; a fixed contact unit provided on one side of the front space and electrically connected to one of a power supply and a load; a movable contact unit installed in the front space (S 1) to be movable and being in contact with the fixed contact unit or being separated from the fixed contact unit; a switching device installed in the rear space (S 2) and operating to allow the movable contact unit to be in contact with the fixed contact unit or to be separated from the fixed contact unit; an operation device installed in the front space (S 1) and the rear space (S 2) and transferring the movable contact unit according to operation of the switching device; and an electrode shaft installed on one side of the upper external box, corresponding to an outside of the front space, wherein the operation device comprises: a first link element comprising an upper link connected to the switching device and a lower link connected to the upper link; a second link element installed to be pivotable around the electrode shaft, one side of the second link element being connected to another side of the lower link to be pivotable; and a third link element with one side connected to another side of the second link element to be pivotable and another side connected to the movable contact unit to be pivotable, characterized in that the upper external box is provided with a shaft mounting groove formed by denting a part of a top surface of the upper external box downwards to allow the electrode shaft to be mounted thereon, wherein the upper external box is provided with a link penetration slot formed by cutting a part of the shaft mounting groove to allow the second link element fixed to the electrode shaft to penetrate thereinto, and wherein the second link element is formed together with the electrode shaft as a single body. a first link element comprising an upper link connected to the switching device and a lower link connected to the upper link; a second link element installed to be pivotable around the electrode shaft, one side of the second link element being connected to another side of the lower link to be pivotable; and a third link element with one side connected to another side of the second link element to be pivotable and another side connected to the movable contact unit to be pivotable, the upper external box is provided with a shaft mounting groove formed by denting a part of a top surface of the upper external box downwards to allow the electrode shaft to be mounted thereon, wherein the upper external box is provided with a link penetration slot formed by cutting a part of the shaft mounting groove to allow the second link element fixed to the electrode shaft to penetrate thereinto, and wherein the second link element is formed together with the electrode shaft as a single body.