PURPOSE: To enable lightening of weight and to enable improving of strength, by a composition wherein a consuming volume of a light aggregate can account for 20 or more volume percent of a core portion, and 70 or more weight percent of the light aggregate may have a particle diameter of 2.5mm or more.

CONSTITUTION: A consuming volume of a light aggregate 23 required to compose a core layer is required to be volume which can account for 20 or more volume percent of a core portion 2. A particle size of the light aggregate 23 is required to be such that 70 or more weight percent of the consuming aggregate 23 has a particle diameter of 2.5mm or more. This causes a fiber material in a surface layer 6 to be prevented from dispersing into a core layer, and permits the mainteance of a high density of a fiber material in the surface layer 6. Thus, the light aggregate is excellent in a shork-proof property, a bending strength, the modulus of elasticity, etc. as well as a lightness, and a compressive strength can also be improved.