To eliminate the necessity of removing wheel shaft detecting pieces at multiple work by arranging a pair of axle detectors in a cutout part formed on a sleeper for placing a rail in a state of floating a bottom surface of the rail.

A cutout part Ca is arranged so that a pair of the part for placing a rail R becomes lower than an upper surface of a sleeper C, that is, a prescribed interval is generated on a bottom surface of the rail R placed on the sleeper C. The depth of the cutout part Ca is decided so that a baseplate 1 can be inserted into the under side of the rail R. An upper surface of the cutout part Ca is decided so as to become parallel to an upper surface of the sleeper C. The rail R is placed on and fixed to an unarranged part of the cutout part Ca f the sleeper C. Axle detectors (d) are arranged by sandwiching the rail R via the baseplate 1. The baseplate 1 is formed in a rectangular shape in a plane shape, a thickness is slightly thinned more than the depth of the cutout part, and the lengthwise directional length is decided so as to project by the prescribed length from both sides of the rail R when crossed in the lengthwise direction of the rail R.