To prevent the occurrence of breakdown and a crack of an inner case by damping impact force, by inserting a travel guide pin pulled out by a top-railed sliding door side magnet into the inner case inserted into an outer case arranged on a floor surface, and forming an inner case upper end part exposed to the floor surface out of a soft material.

An outer case 3 is embedded under a floor surface by preventing slipping-out via a locking projection 19 on the outer periphery. A cylindrical inner case 4 sufficiently shorter than the outer case 3 is fitted inside this outer case 3 so as to be freely inserted and extracted, and a travel guide pin 7 is inserted into the inner case 4 and the outer case 3 so as to be freely liftable. In a state of most pulling up the travel guide pin 7 by being magnetically installed on the top-railed sliding door 5 side, a lower end magnet 26 of the travel guide pin 7 is magnetically installed on a holding magnet 27 to hold a state of pulling up the travel guide pin 7. An upper end part of the inner case 4 exposed to the floor surface is formed of a soft material 8, and even if a falling hard article unexpectedly collides with the floor surface, impact force is damped by this soft material 8.