A switchable magnetic chuck has a magnet assembly slidably arranged in a casing to magnetize and demagnetize a top plate disposed in an open side of the casing. The top plate is provided with main pole members and auxiliary pole members which are arranged alternatively in the sliding direction of the magnet assembly, separated respectively by a non-magnetic separator disposed between them. To smoothen the change-over operation, enhance magnetic force under the magnetized condition and reduce residual attraction under the demagnetized condition, each main pole member is formed on the side facing the magnet assembly with an extension extending between the auxiliary pole member adjacent thereto and the magnet assembly.