Abstract: In the methionine synthesis involving a hydantoin reaction in its steps, a process for recovering and reusing the useful components in the waste gas by (1) washing the waste gas generated in the hydantoin synthesis reactor with methylmercaptopropionaldehyde (hereinafter referred to as MA) and returning the washing into the MA synthesis reactor system and/or (2) washing the waste gas with an aqueous solution of a compound substantially constituted of ammonia and carbon dioxide, i.e. the starting material of hydantoin synthesis, or with the liquid reaction mixture of methionine containing hydantoin (hereinafter referred to as MH) and returning the washing into the MH synthesis reactor system.