A reactor, which heats a slurry raw material including a solid component, including a reaction tube, wherein the slurry raw material is continuously fed from an end of the reaction tube relative to the central axial direction of the reaction tube so as to be flown toward another end of the reaction tube to be heated; and plural perforated plates arranged in the reaction tube so as to separate the inside of the reaction tube into plural compartments. The reactor satisfies the relations (½)·D′≦D