A gas reaction system is disclosed which comprises a vaporizer (230) for generating a reaction gas by vaporizing a liquid material and a reaction chamber (221A) wherein the reaction gas is reacted. The vaporizer (230) is integrally formed with a component member which defines the reaction chamber (221A). The reaction gas generated in the vaporizer (230) is directly introduced into the reaction chamber (221A). The vaporization chamber (232) of the vaporizer (230) is a space between an upper plate (230A) and a cap (230B) attached to the upper surface of the upper plate (230A). A narrow passage (233) is formed between the cap (230B) and the upper plate (230A) which passage (233) communicates with the vaporization chamber (232).