To promote bond ability of a cement mortar itself to a foam synthetic resin material in comparison with the conventional composite building material, prevent occurrence of separation, crack, etc., in a new composite building material, and to have the composite building material excellent in strength.

A plurality of groove threads 6 are arranged in parallel on at least one side face of both front and rear faces of a foam synthetic resin plate material layer 2 to recess, and meshy groove thread lathes 3 corresponding to sizes of the groove threads are arranged to the insides of the groove threads 6. After the groove threads are filled with cement mortar so as to embed the groove thread lathes 3, meshy surface lathes 4 are stretched on the whole one side face to laminate cement mortar again, and a composite building material 1 is monolithically formed of the foam synthetic resin plate material layer 2 and a cement mortar layer 5. Each of the groove threads 6 is recessed and is formed in the shape of an approximately dovetail slot, and the composite building material is so constituted that the groove thread lathes 3 are arranged in the groove inside sections of the groove threads 6.