To prevent a warf or a deformation even when this composite board is used for a wall face and make a large-sized artificial marble board usable by forming a recessed part and a projecting part in a plate respectively in mutually opposite sides of a marble thin plate when the plate is stuck to the rear face of the artificial marble thin plate with an adhesive.

This composite board 1 is constituted of an artificial marble thin plate 2, a plate 3 made of a woody plate such as a particle board or the like, and an adhesive 4 such as a solvent type acrylic adhesive, an emulsion type acrylic adhesive, etc., combining these plates. When the plate 3 is stuck to the rear face of the artificial marble thin plate 2, the plate 3 and the marble thin plate 2 are mutually stuck together so as to shift each other to form a projecting part 31 and a recessed part 32. In this case, when the width of the projecting part 31 is α, and the width of the recessed part 32 is β, these are formed as α>β, and the front end of the projecting part 31 is butted against the front end of the recessed part 31 to fix the composite boards 1 one after another. In this way, a gap having a width of α-β is formed between adjacent artificial marble thin plates 2, and the gaps are used as joints.