A gas analysis device comprising a mixed gas generation mechanism that comprises a heating furnace that is configured to heat a crucible where a sample is put while introducing a carrier gas to be an object substance not to be measured, to generate a sample gas that contains an object substance to be measured by vaporizing at least a part of the sample and to discharge a mixed gas comprising the carrier gas and the sample gas, a quadrupole mass spectrometer that has a measurement space into which the mixed gas generated by the mixed gas generation mechanism is introduced and that is configured to conduct a quantitative analysis on the object substance to be measured by giving energy to the object substance to be measured and the object substance not to be measured in the measurement space, and a pressure control mechanism that is configured to adjust a pressure in the measurement space, wherein the pressure control mechanism is configured to adjust the pressure in the measurement space so as to make the energy that is given to the object substance not to be measured in the measurement space equal or less than the lower limit value of the ionization energy of the object substance not to be measured and more than or equal to the lower limit value of the ionization energy of the object substance to be measured, wherein when the pressure in the measurement space is decreased, a speed of an electron to ionize the substance is fast and the energy given to the substance increases and when the pressure in the measurement space is increased, a speed of an electron to ionize the substance is slow and the energy given to the substance decreases. a mixed gas generation mechanism that comprises a heating furnace that is configured to heat a crucible where a sample is put while introducing a carrier gas to be an object substance not to be measured, to generate a sample gas that contains an object substance to be measured by vaporizing at least a part of the sample and to discharge a mixed gas comprising the carrier gas and the sample gas, a quadrupole mass spectrometer that has a measurement space into which the mixed gas generated by the mixed gas generation mechanism is introduced and that is configured to conduct a quantitative analysis on the object substance to be measured by giving energy to the object substance to be measured and the object substance not to be measured in the measurement space, and a pressure control mechanism that is configured to adjust a pressure in the measurement space, wherein the pressure control mechanism is configured to adjust the pressure in the measurement space so as to make the energy that is given to the object substance not to be measured in the measurement space equal or less than the lower limit value of the ionization energy of the object substance not to be measured and more than or equal to the lower limit value of the ionization energy of the object substance to be measured, wherein when the pressure in the measurement space is decreased, a speed of an electron to ionize the substance is fast and the energy given to the substance increases and when the pressure in the measurement space is increased, a speed of an electron to ionize the substance is slow and the energy given to the substance decreases.