A U-shaped magnetic plate is positioned on the side of a movable contact shoe opposite to a fixed contact shoe, and the tips of the respective legs of the magnetic plate face via a gap the movable contact shoe in a closed condition of the movable contact shoe. A magnetic plate is attached to the surface of the movable contact shoe at the side of the fixed contact, so that the magnetic plates form a closed magnetic circuit surrounding the movable contact shoe. If a high current, such as short circuit current, flows, a magnetic flux passing through the closed magnetic circuit induces an attractive force between the upper end surfaces of the U-shaper magnetic plate and the end surface of the magnetic plate. Accordingly, the movable contact shoe is rapidly opened to quickly interrupt the current, before a signal from an overcurrent trip apparatus operates an opening and closing mechanism. The opening speed of the movable contact shoe is increased when high current flows.