Abstract: A magnetic circuit device suitable for use in a magnetic catch having a switching function, a slide switch or a sensor for detecting locations of a movable member has been found. The magnetic circuit device comprises a main permanent magnet (1) having a pair of magnetic poles (N, S) on opposite faces, a pair of yoke pieces (2) lying on the faces, a movable piece (4) made of magnetic material capable of engaging with first ends of the yoke pieces (2), and a sub-permanent magnet (6) movably disposed near second ends of the yoke pieces (2) opposite to the first edges so that when the movable piece (4) is attracted to the first ends, the sub-permanent magnet (6) is attracted to the second ends, and when the movable piece (4) is made break away from the first ends, the sub-permanent magnet (6) breaks away from the second ends. Movement of the sub-magnet (6) can be utilized to control electrical connection of contacts (8A, 8B) of a switching mechanism.