Abstract: Provided is a composite building material which is more improved in the bonding capability of a cement mortar material itself with a foamed synthetic resin plate layer, furthermore prevents the occurrences of exfoliations and cracks and has higher strength compared with conventional composite materials. Plural concave grooves are formed in parallel on at least one of the surface and backface of a foamed synthetic resin plate layer and a net-like groove lathing material corresponding to the size of each groove is arranged within each groove. A cement mortar material is filled in the groove so as to bury the groove lathing material, a net-like surface lathing material is then stretched over all of one surface and a cement mortar material is further laminated to form a composite building material in which the foamed synthetic resin plate layer and the cement mortar layer are integrated with each other.