PURPOSE: To simplify the changeover of measuring modes and to simultaneously execute measurements in both modes during one gas geenration period in a gas analytical apparatus which is provided with two measuring modes as a trap-mode measurement and a direct-mode measurement.

CONSTITUTION: A gas analytical apparatus is provided with a gas-trap device 24 which traps a gas generated from a sample 8, with a gas chromatograph 31 and with a mass spectometer 4. In the gas analytical apparatus, a four-way valve 28 and a six-way valve 29 are changed over, a direct-mode gas conveyance passage A, a gas trap introduction passage B and a gas trap discharge passage C are selected properly and connected mutually, and a direct-mode measurement and a trap-mode measurement can be performed continuously during one gas generation period.