A method and a device for measuring detailed decomposition rate characteristic of special material gas such as silane gas. After inert gas is supplied from a gas purifier (1) (a first gas-supply source) into a reaction pipe (20) to provide a high purity atmosphere in said pipe, the inside of said pipe is baked by heaters (25 to 27). Then, the inside of the reaction pipe (20) is kept in an atmosphere at a specified temperature and special material gas with a specified purity is supplied from a bomb (10) (a second gas-supply source) into the reaction pipe at a fixed flow rate. When the inside of the reaction pipe (20) reaches a specified terminal temperature, a decomposition rate of the special material gas at said temperature is measured by a gas chromatography (12) by, for example, extracting a part of gas in the reaction pipe. Afterward, a flow rate of the special material gas is successively varied and a decomposition rate at every time of variation is measured. Further, an atmosphere inside the reaction pipe (20) is successively set to one at another specified temperature for measuring a decomposition rate at every temperature. With an impurity concentration in the special gas varied, said measurement is repeated.