An overcurrent circuit breaker is housed in an approximately square housing (1) made of an insulating material and composed of two parts, a chassis (2) and a closure (3). A switching gear which guides the displacement of a contact bridge (13) in a direction parallel to the side walls (4, 5) of the housing parts (2, 3) is arranged between the two parallel side walls (4, 5) of the housing parts (2, 3). The contact bridge extends in a substantially perpendicular direction to the side walls (4, 5), and in the switching-on position is positioned between two contacts (14, 15) fixed to the housing and located on both sides of the plane of displacement of the switching gear. The fixed contacts (14, 15) are flat, similar to knife blades. They lie on the side walls (4, 5) and form contact points (16, 17) with their narrow edges which face the contact bridge (13) like the cutting edge of a knife. The backs (18, 19) of the fixed contacts (14, 15) opposite to the contact points (16, 17) lie on a flank wall (12) of the housing (1). The fixed contacts (14, 15) are thus positioned in a particularly space-saving manner in corners of the housing (1).