A gas purifier of the present invention includes a purifier in which a gas-purifying agent is packed, wherein a gas is fed into the purifier, and impurities in the gas are removed by a thermal swing adsorption method, in which an amount A of the gas-purifying agent is determined such that an impurities-removing capacity possessed by half of the amount A of the gas-purifying agent is equal to the total amount of impurities in the gas to be purified in one purification step, and the amount of the gas-purifying agent packed in the purifier is the amount A or more.