A gas purification apparatus, comprising: a housing to which a gas is introduced; a filter portion for removing an impure substance in the gas from the gas, the filter portion being disposed inside the housing; and a gas purification agent (9, 9a, 9b) for removing a removal target substance in the gas from the gas, the gas purification agent (9, 9a, 9b) being disposed, inside the housing, in a space at a downstream side of the filter portion with respect to a flow of the gas, wherein the housing includes, above the filter portion, a gas purification agent supply pipe connected tc the housing and a supply valve disposed in the supply pipe for supplying the gas purification agent (9, 9a, 9b) into the housing, wherein the filter portion includes a filter element having a tubular shape and a hollow portion, the filter element being disposed so as to protrude from the space at the downstream side toward a space at un upstream side of the filter portion with respect to the flow of the gas, the gas purification agent (9, 9a, 9b) being disposed in the hollow portion of the filter element, and a support plate for supporting the filter element, the support plate being disposed so as to spread along a horizontal direction and having an opening of substantially the same diameter as the tubular shape of the filter element, the upper end portion of the filter element being attached to the opening of the support plate and the filter element extending downward from the support plate, the support plate being configured to store a backup gas purification agent (9, 9b) above said gas purification agent (9, 9a), the gas purification apparatus further comprising: a discharge port for discharging the gas purification agent (9, 9a, 9b) from the hollow portion of the filter element, the discharge port being disposed on the filter element, and a discharge chamber through which the gas purification agent (9, 9a, 9b) is to be discharged from the hollow portion, wherein the discharge chamber and the space at the upstream side are separated in the internal space of the housing by a separation plate that extends in the horizontal direction in the vicinity of the lower end portion of the filter element. a housing to which a gas is introduced; a filter portion for removing an impure substance in the gas from the gas, the filter portion being disposed inside the housing; and a gas purification agent (9, 9a, 9b) for removing a removal target substance in the gas from the gas, the gas purification agent (9, 9a, 9b) being disposed, inside the housing, in a space at a downstream side of the filter portion with respect to a flow of the gas, wherein the housing includes, above the filter portion, a gas purification agent supply pipe connected tc the housing and a supply valve disposed in the supply pipe for supplying the gas purification agent (9, 9a, 9b) into the housing, wherein the filter portion includes a filter element having a tubular shape and a hollow portion, the filter element being disposed so as to protrude from the space at the downstream side toward a space at un upstream side of the filter portion with respect to the flow of the gas, the gas purification agent (9, 9a, 9b) being disposed in the hollow portion of the filter element, and a support plate for supporting the filter element, the support plate being disposed so as to spread along a horizontal direction and having an opening of substantially the same diameter as the tubular shape of the filter element, the upper end portion of the filter element being attached to the opening of the support plate and the filter element extending downward from the support plate, the support plate being configured to store a backup gas purification agent (9, 9b) above said gas purification agent (9, 9a), the gas purification apparatus further comprising: a discharge port for discharging the gas purification agent (9, 9a, 9b) from the hollow portion of the filter element, the discharge port being disposed on the filter element, and a discharge chamber through which the gas purification agent (9, 9a, 9b) is to be discharged from the hollow portion, wherein the discharge chamber and the space at the upstream side are separated in the internal space of the housing by a separation plate that extends in the horizontal direction in the vicinity of the lower end portion of the filter element. a discharge port for discharging the gas purification agent (9, 9a, 9b) from the hollow portion of the filter element, the discharge port being disposed on the filter element, and a discharge chamber through which the gas purification agent (9, 9a, 9b) is to be discharged from the hollow portion, wherein the discharge chamber and the space at the upstream side are separated in the internal space of the housing by a separation plate that extends in the horizontal direction in the vicinity of the lower end portion of the filter element.