PURPOSE: To uniformize the concentration gradients of material gas on the surface and periphery of a crystal substrate thereby to easily obtain a thin film crystal having a uniform thickness by supplying gas to be reacted substantially perpendicularly from below the surface of the substrate to be held by a susceptor.

CONSTITUTION: Reaction gas which contains material gas is supplied from a reaction gas supply port 9 of a reaction furnace body 1 toward the surface of a crystal substrate 7 held on the lower surface of a susceptor 5. The supplied reaction gas is guided by a diffuser 13 to arrive at the substrate 7, heated to a predetermined temperature by a high frequency induction heater 17, reacted on the surface of the substrate 7 rotated by a rotational shaft 3, decomposed to become a crystal and grown on the substrate 7. The reaction gas which is finished to react and decompose on the substrate rises in a space 15 between the peripheral edge 5a of the susceptor 5 and the inner wall 1b of the body 1, and is led from an outlet 11 at the top of the body 1 to an exhaust gas processor.