PURPOSE: To provide a composite panel suitable as a civil engineering material or a building material such as a concrete form, a floor material, a wall material or a ceiling material.

CONSTITUTION: Adjacent reinforcing ribs 11 are formed between the upper and rear surface plates of a hollow base panel 1 molded from a molding material prepared by adding 5-50wt.% of an inorg. filler to polyolefinic plastic based on polypropylene by extrusion molding so as to be inclined at an angle θ(45°≤θ<90°) in mutually opposite directions and a foamed plastic layer 2 with a foaming magnification of 3-20 times is fused to the inner surfaces of the upper and rear surface plates of the hollow base panel 1 and the surfaces of the reinforcing ribs 11 to be integrally molded. Therefore, the composite panel thus produced is excellent in productivity by extrusion molding and nailing processing properties and light in wt. and has high heat insulating properties and required mechanical strength.