To form accurately a required irregular pattern on a surface of a formed product while the whole formed product can be of course thoroughly uniformly dehydrated.

The forming device 1a is so constituted that an irregular pattern forming part is provided to a part of a mold surface, and a raw material is formed by compression with top and bottom forces dehydrating excessive water content in the raw material in the mold from the mold surface including the irregular pattern forming part via a suction dehydrating means. In this case, the irregular pattern forming part is formed to a porous synthetic resin made mold material 53, the mold material 53 is freely detachably fixed to a plate material 52 so as to cover a part of the plate material 52 through which many drainage holes 52a interconnected to a suction dehydrating means are bored, and the remaining part of the plate material 52 constitutes a part of the mold surface.