A gas chromatograph mass spectrometer including: an oven for gas chromatography, a mass spectrometry part, a sample vaporization chamber, a gas supply part, and a control part. When the replacement operation of a septum or a glass insert begins, the control part controls the gas supply part so that the flow rate of a carrier gas has a constant pressure and then controls heaters so that temperature is equal to or less than a temperature at which replacement is possible. When temperature becomes equal to or less than the temperature at which replacement is possible and the replacement operation of the septum or the glass insert is completed, the control part controls the heaters so that the temperature is an analysis temperature and controls the gas supply part to employ the same control method for the flow rate of the carrier gas as that before the replacement operation is begun.