To provide a resin building material for floor, improving failures such as push-up and cleavage of a hard resin material due to heat expansion and heat shrinkage of the hard resin material in a temperature zone of 0 to 40°C, and enhancing convenience during construction.

A clearance, which can permit a thermal extension/retraction of a hard resin layer in a temperature zone of 0 to 40°C with respect to a direction orthogonal to a side face having a fitting structure on a flat surface of a resin building material for floor, is provided on a surface side of the building material when it is fitted. The building material has a projection on its lateral face side facing a lateral side having a downward projection of another building material. The cross-sectional shape of the projection is so formed that the top face of the projection is provided substantially in parallel with a floor surface and the intersecting angle of a line drawn from the top face of the projection toward the peak of the projection and a line drawn from the underside of the projection toward the peak of the projection is within a range of 35 to 55°.