PURPOSE: To obtain the composite material having a reinforcing effect and facilitating secondary working by subjecting a mixture, where aluminum borate and Al-type metal are uniformly dispersed, to heating treatment and decomposing aluminum borate into alumina and metallic boron.

CONSTITUTION: Aluminum borate and Al-type metal in a molten state are subjected, e.g. to mixing by agitation, by which a mixture where aluminum borate is uniformly dispersed in Al-type metal is prepared. This mixture is heated up to 500-900°C under an inert atmosphere, by which aluminum borate is decomposed by reduction. By this method, aluminum borate is decomposed into alumina and metallic boron, and the composite material where the fine grains of alumina are dispersed in the Al-type metal can be obtained.