A preprocessing apparatus for gas analysis comprising: a water trapping section configured to trap water from sample water, in which a target gas to be extracted is dissolved, to extract a to-be-purified target gas; and an adsorption section configured to adsorb a non-target gas for extraction in the to-be-purified target gas, characterized in that: the water trapping section includes: a first cold trap configured to cool the sample water, which has flowed in a first trap path, at a first temperature at which the target gas is not condensed, thereby removing a liquid phase to obtain a to-be-purified target gas containing water vapor, and to establish a first vacuum environment in the first trap path, and a second cold trap configured to further cool the to-be-purified target gas containing water vapor, which has been purified by the first cold trap and then has flowed in a second trap path, at a second temperature at which the water vapor is condensed but the target gas is not condensed and which is lower than the first temperature, thereby removing the water vapor to obtain the to-be-purified target gas not containing water vapor, and to establish a second vacuum environment in the second trap path such that the pressure in the second vacuum environment is lower than that in the first vacuum environment; and the adsorption section includes at least one getter that adsorbs the non-target gas for extraction from the to-be-purified target gas in a getter placement path and establishes a vacuum environment in the getter placement path such that the pressure in the getter placement path is lower than that in the water trapping section. a water trapping section configured to trap water from sample water, in which a target gas to be extracted is dissolved, to extract a to-be-purified target gas; and an adsorption section configured to adsorb a non-target gas for extraction in the to-be-purified target gas, characterized in that: the water trapping section includes: a first cold trap configured to cool the sample water, which has flowed in a first trap path, at a first temperature at which the target gas is not condensed, thereby removing a liquid phase to obtain a to-be-purified target gas containing water vapor, and to establish a first vacuum environment in the first trap path, and a second cold trap configured to further cool the to-be-purified target gas containing water vapor, which has been purified by the first cold trap and then has flowed in a second trap path, at a second temperature at which the water vapor is condensed but the target gas is not condensed and which is lower than the first temperature, thereby removing the water vapor to obtain the to-be-purified target gas not containing water vapor, and to establish a second vacuum environment in the second trap path such that the pressure in the second vacuum environment is lower than that in the first vacuum environment; and the adsorption section includes at least one getter that adsorbs the non-target gas for extraction from the to-be-purified target gas in a getter placement path and establishes a vacuum environment in the getter placement path such that the pressure in the getter placement path is lower than that in the water trapping section. the water trapping section includes: a first cold trap configured to cool the sample water, which has flowed in a first trap path, at a first temperature at which the target gas is not condensed, thereby removing a liquid phase to obtain a to-be-purified target gas containing water vapor, and to establish a first vacuum environment in the first trap path, and a second cold trap configured to further cool the to-be-purified target gas containing water vapor, which has been purified by the first cold trap and then has flowed in a second trap path, at a second temperature at which the water vapor is condensed but the target gas is not condensed and which is lower than the first temperature, thereby removing the water vapor to obtain the to-be-purified target gas not containing water vapor, and to establish a second vacuum environment in the second trap path such that the pressure in the second vacuum environment is lower than that in the first vacuum environment; and a first cold trap configured to cool the sample water, which has flowed in a first trap path, at a first temperature at which the target gas is not condensed, thereby removing a liquid phase to obtain a to-be-purified target gas containing water vapor, and to establish a first vacuum environment in the first trap path, and a second cold trap configured to further cool the to-be-purified target gas containing water vapor, which has been purified by the first cold trap and then has flowed in a second trap path, at a second temperature at which the water vapor is condensed but the target gas is not condensed and which is lower than the first temperature, thereby removing the water vapor to obtain the to-be-purified target gas not containing water vapor, and to establish a second vacuum environment in the second trap path such that the pressure in the second vacuum environment is lower than that in the first vacuum environment; and the adsorption section includes at least one getter that adsorbs the non-target gas for extraction from the to-be-purified target gas in a getter placement path and establishes a vacuum environment in the getter placement path such that the pressure in the getter placement path is lower than that in the water trapping section.