A molded case circuit beaker comprising: an upper outer casing; a lower outer casing; wherein the molded case circuit breaker further comprises: an intermediate insulation barrier having one side concave and integrally formed with the upper outer casing and for partitioning a lower compartment and an upper compartment, wherein the upper compartment is formed by the intermediate insulation barrier and walls of the upper outer casing present upper than the intermediate insulation barrier, and wherein the lower compartment is formed by the intermediate insulation barrier, the upper outer casing present below the intermediate insulation barrier and walls of the lower outer casing; a main circuit unit (21, 22, 23) present in the lower compartment and configured to open or close a circuit by having a stationary contactor and a movable contactor rotatable to contact with or separated from the stationary contactor; a switching mechanism present in the upper compartment and having an open position where the switching mechanism is connected to the main circuit unit to drive the main circuit unit to open a circuit, and a closing position where the switching mechanism drives the main circuit unit to close a circuit; characterized by an instantaneous trip mechanism (1, 2, 3) present in the upper compartment, and operated by an electromagnetic attraction directly generated by a fault current on a circuit so as to trigger the switching mechanism to the open position; and in that the intermediate insulation barrier installed commonly for three phases between the upper compartment and the lower compartment for electrical insulation by partitioning the lower compartment having the main circuit unit and the upper compartment having the instantaneous trip mechanism and the switching mechanism. an upper outer casing; a lower outer casing; an intermediate insulation barrier having one side concave and integrally formed with the upper outer casing and for partitioning a lower compartment and an upper compartment, wherein the upper compartment is formed by the intermediate insulation barrier and walls of the upper outer casing present upper than the intermediate insulation barrier, and wherein the lower compartment is formed by the intermediate insulation barrier, the upper outer casing present below the intermediate insulation barrier and walls of the lower outer casing; a main circuit unit (21, 22, 23) present in the lower compartment and configured to open or close a circuit by having a stationary contactor and a movable contactor rotatable to contact with or separated from the stationary contactor; a switching mechanism present in the upper compartment and having an open position where the switching mechanism is connected to the main circuit unit to drive the main circuit unit to open a circuit, and a closing position where the switching mechanism drives the main circuit unit to close a circuit; characterized by an instantaneous trip mechanism (1, 2, 3) present in the upper compartment, and operated by an electromagnetic attraction directly generated by a fault current on a circuit so as to trigger the switching mechanism to the open position; and in that the intermediate insulation barrier installed commonly for three phases between the upper compartment and the lower compartment for electrical insulation by partitioning the lower compartment having the main circuit unit and the upper compartment having the instantaneous trip mechanism and the switching mechanism.