A circuit breaker, comprising: a cradle; a circuit breaker main body; a carriage, comprising: a girder part constituting a fixed body; a carriage main body part forming a moving body to which the circuit breaker main body is coupled; a lead screw having one end rotatably coupled with the girder part; and a nut assembly fixed to the carriage main body part and engaged with the lead screw to allow the carriage main body part to be moved when the lead screw is rotated; and a block assembly selectively coupled to the lead screw at the girder part side to prevent rotation of the lead screw and moving in a lengthwise direction of the lead screw by a withdrawal and insertion handle applying an additional external force from the exterior, wherein the movement in the lengthwise direction allows the withdrawal and insertion handle to be coupled with the lead screw; and an apparatus configured to prevent the circuit breaker main body from being withdrawn and inserted, respectively, comprising: a push racking plate disposed to be parallel to the lead screw, installed to be movable in a lengthwise direction of the lead screw with respect to the carriage main body part and coupled to the block assembly; a racking interlock pin installed at the carriage main body part such that it is movable in a direction perpendicular to a direction in which the push racking plate (151, 55) is moved, and coupled with the carriage main body part; and an interlocking unit provided at one side of the racking interlock pin and configured to prevent the circuit breaker main body from being withdrawn from the cradle by limiting an ascending movement of the racking interlock pin by rotating toward the racking interlock pin when the circuit breaker performs a closing operation and thus by restricting a movement of the push racking plate (151, 55) in a lengthwise direction by the racking interlock pin. a cradle; a circuit breaker main body; a carriage, comprising: a girder part constituting a fixed body; a carriage main body part forming a moving body to which the circuit breaker main body is coupled; a lead screw having one end rotatably coupled with the girder part; and a nut assembly fixed to the carriage main body part and engaged with the lead screw to allow the carriage main body part to be moved when the lead screw is rotated; and a block assembly selectively coupled to the lead screw at the girder part side to prevent rotation of the lead screw and moving in a lengthwise direction of the lead screw by a withdrawal and insertion handle applying an additional external force from the exterior, wherein the movement in the lengthwise direction allows the withdrawal and insertion handle to be coupled with the lead screw; and a girder part constituting a fixed body; a carriage main body part forming a moving body to which the circuit breaker main body is coupled; a lead screw having one end rotatably coupled with the girder part; and a nut assembly fixed to the carriage main body part and engaged with the lead screw to allow the carriage main body part to be moved when the lead screw is rotated; and a block assembly selectively coupled to the lead screw at the girder part side to prevent rotation of the lead screw and moving in a lengthwise direction of the lead screw by a withdrawal and insertion handle applying an additional external force from the exterior, wherein the movement in the lengthwise direction allows the withdrawal and insertion handle to be coupled with the lead screw; and an apparatus configured to prevent the circuit breaker main body from being withdrawn and inserted, respectively, comprising: a push racking plate disposed to be parallel to the lead screw, installed to be movable in a lengthwise direction of the lead screw with respect to the carriage main body part and coupled to the block assembly; a racking interlock pin installed at the carriage main body part such that it is movable in a direction perpendicular to a direction in which the push racking plate (151, 55) is moved, and coupled with the carriage main body part; and an interlocking unit provided at one side of the racking interlock pin and configured to prevent the circuit breaker main body from being withdrawn from the cradle by limiting an ascending movement of the racking interlock pin by rotating toward the racking interlock pin when the circuit breaker performs a closing operation and thus by restricting a movement of the push racking plate (151, 55) in a lengthwise direction by the racking interlock pin. a push racking plate disposed to be parallel to the lead screw, installed to be movable in a lengthwise direction of the lead screw with respect to the carriage main body part and coupled to the block assembly; a racking interlock pin installed at the carriage main body part such that it is movable in a direction perpendicular to a direction in which the push racking plate (151, 55) is moved, and coupled with the carriage main body part; and an interlocking unit provided at one side of the racking interlock pin and configured to prevent the circuit breaker main body from being withdrawn from the cradle by limiting an ascending movement of the racking interlock pin by rotating toward the racking interlock pin when the circuit breaker performs a closing operation and thus by restricting a movement of the push racking plate (151, 55) in a lengthwise direction by the racking interlock pin.