PURPOSE: To manufacture metal-reinforced synthetic wood which is resistant against humidity, excellent in strength, high in quality and inexpensive by injecting synthetic resin material into the die of a molding machine wherein a metal body assembled as a rib material is set, and by performing reaction injection molding.

CONSTITUTION: A frame composed of transverse pieces 1 and longitudinal pieces 2 which are made of high-carbon special steel having the high carbon content is set in a molding die, for instance, and synthetic resin material of polyol and isocyanate is injected into this die. Then, polyaddition reaction, foaming reaction and crosslinking reaction are effected in the die so that molding is performed with the frame covered with a cured polyurethane foamed body 30, and thereby the desired metal-reinforced synthetic wood is manufactured.