To provide a wooden plastic-made sleeper, which has sufficient strength with excellent heat insulating property, and be produced through a simple process by effectively using waste materials generated in a woodworking factory or the like as resources while reducing the production cost.

The building sleeper 1 having a substantially square cross section which is composed of a hard fiber material obtained by kneading a cellulose-based powder with a thermoplastic binder followed by extrusion molding include stop grooves 11a and 11a formed on the outside surfaces of side walls 11 and 11 to be paired at the same level, and an upward hooking protruding part 11b integrally provided on the lower edge part of the stop groove 11a, so that a receiving plate part 21 of a receiving seat member 2 can be substantially horizontally supported by fitting a clip groove 22 to the hooking protruding part 11b. In such sleepers 11 set adjacently while being supported by floor posts 3, a heat insulating panel P can be erected between both the members by placing edge parts of the panel P on the upper surfaces of the receiving plate parts 21 and 21.