PURPOSE: To facilitate synchronization of switching timing of a displacement sensor coil to an electromagnet by forming the length of the electromagnet in a rail direction integer number times of the length of the coil, and bringing both ends of the plurality of the coils and both ends of the electromagnet into coincidence with the rail direction.

CONSTITUTION: Displacement sensors 6a - 6d are switched upon movement of a movable element 2, and a gap displacement between the sensor target 8 of the element 2 and a rail is detected. On the other hand, electromagnets 7a, 7b are formed in a rectangular shape similarly to the sensors 6a - 6d, aligned and disposed on the rail. Here, the lengths of the electromagnets 7 corresponding to the sensors 6 in the rail direction are set integer number times of the length of the sensors, and both end faces are brought into coincidence. Thus, synchronization of the switching timing of the sensors 6 to the electromagnets 7 is facilitated to smoothly travel the element 2.