The present invention is a suction box for dehydrating the intermediate form of an inorganic building board during manufacturing or dehydrating the material slurry which is eventually molded into inorganic building boards. The suction box can change the open area ratio of dehydration space easily and can be used with pressure forming machines. The suction box has a frame member with a bottom plate, plural bridge members, plural spacer members and a pair of fixture members. The open area ratio is the space formed between each bridge member, including an interval with the bridge member and an outer frame portion in the frame member with the bottom plate.