PURPOSE: To provide an extrusion-molded plastic panel unit wherein defects of a panel prepd. by conventional technologies are improved and without substantially increasing thicknesses of main surfaces and thicknesses of inner ribs and surfaces, possibility of resistance to a pressure applied on the panel is increased.

CONSTITUTION: An extrusion-molded panel unit for building has at least two sheet-like main surfaces connected with each other by a plurality of ribs 8 and separated by a distance and intermediate surfaces 10 dividing the space formed by the main surfaces into a plurality of sub-spaces 12. In the crosssection, the sheet-like main surfaces, the ribs and the intermediate surfaces form a truss-like structure and at least three sub-spaces extending substantially vertically to the main surfaces and crossing the planes connecting the main surfaces are provided.