An overcurrent tripping device which detects an overcurrent flowing through a main circuit of a circuit breaker and actuates a tripping mechanism of the circuit breaker in a closed state, the overcurrent tripping device comprising: a conductor connected to the main circuit; a fixed core surrounding the conductor along a magnetic field generated around the conductor by a current flowing through the conductor, the fixed core being partially open so as to obliquely cut the magnetic field; a movable core having a magnetic gap between the fixed core and the movable core at a position where the fixed core is open, the movable core being disposed such that the movable core is movable in a direction in which the magnetic gap is reduced by an electromagnetic force generated when an overcurrent flows through the conductor; a rod penetrating an inside of the fixed core through a location where the fixed core is open and a portion of the fixed core facing the location where the fixed core is open, the rod being linked to the tripping mechanism at an end projecting to the outside of the fixed core, the rod being fixed to the movable core in a bar shape; characterised in that the overcurrent tripping device comprises: a lever connected at one end thereof to the rod, connected at another end thereof to a spring provided outside the fixed core, and configured to rotate about a fulcrum provided between respective connection portions and transmit a load force of the spring to the movable core, wherein the fixed core has a gap perpendicular to a direction in which the conductor extends, and, in the gap, a plate-shaped fixed core guide provided with a groove along a direction in which the rod extends is fixed and the lever is disposed, and the movable core has a gap perpendicular to the direction in which the conductor extends, and a plate-shaped movable core guide which is partially interposed in the groove and slides in the direction in which the rod extends is fixed in the gap. a conductor connected to the main circuit; a fixed core surrounding the conductor along a magnetic field generated around the conductor by a current flowing through the conductor, the fixed core being partially open so as to obliquely cut the magnetic field; a movable core having a magnetic gap between the fixed core and the movable core at a position where the fixed core is open, the movable core being disposed such that the movable core is movable in a direction in which the magnetic gap is reduced by an electromagnetic force generated when an overcurrent flows through the conductor; a rod penetrating an inside of the fixed core through a location where the fixed core is open and a portion of the fixed core facing the location where the fixed core is open, the rod being linked to the tripping mechanism at an end projecting to the outside of the fixed core, the rod being fixed to the movable core in a bar shape; characterised in that the overcurrent tripping device comprises: a lever connected at one end thereof to the rod, connected at another end thereof to a spring provided outside the fixed core, and configured to rotate about a fulcrum provided between respective connection portions and transmit a load force of the spring to the movable core, wherein the fixed core has a gap perpendicular to a direction in which the conductor extends, and, in the gap, a plate-shaped fixed core guide provided with a groove along a direction in which the rod extends is fixed and the lever is disposed, and the movable core has a gap perpendicular to the direction in which the conductor extends, and a plate-shaped movable core guide which is partially interposed in the groove and slides in the direction in which the rod extends is fixed in the gap. a lever connected at one end thereof to the rod, connected at another end thereof to a spring provided outside the fixed core, and configured to rotate about a fulcrum provided between respective connection portions and transmit a load force of the spring to the movable core, wherein the fixed core has a gap perpendicular to a direction in which the conductor extends, and, in the gap, a plate-shaped fixed core guide provided with a groove along a direction in which the rod extends is fixed and the lever is disposed, and the movable core has a gap perpendicular to the direction in which the conductor extends, and a plate-shaped movable core guide which is partially interposed in the groove and slides in the direction in which the rod extends is fixed in the gap.