PURPOSE: To make the reinforcing effect multi-axial, by layering a pluality of sheets that are arranged first-dimensionally or second-dimensionally, on fiber group that consist of a resin and a foaming agent whose thermosetting starting temperature is higher than the foaming starting temperature, and subjecting them to heat treatment.

CONSTITUTION: The short fiber groups 2 are mixed with the resin 1 and the foaming agent, and are made into a sheet, and such sheets are put one above the other to form a laminate. In this case, the short fibers are diffused second- dimensionally, and when viewed in cross section, they would look first-dimenional. When the laminate is subjected to heat treatment, the foaming agent is decomposed to yield a gas to form bubbles 3 in the resin. It is preferable that the mixing ratio of the resin, the foaming agent and the fibers is generally such that 2W10wt% foaming agent is to 100wt% resin and 40W120vol% of the fiber group is to 100vol% of the resin.