To improve humidifying performance by improving heat-insulating property of a gas humidifier.

The gas humidifier 1 is provided with hollow fiber membranes and a housing 2 for bundling and holding fiber membranes. Gases having moisture content different from each other are supplied to the inside and outside of the membrane to carry out moisture-exchange between the gases. A jacket 3 surrounds the outer periphery of the housing 2, and has an inlet 17 and an outlet 18 for a secondary gas flowing around the outer periphery of the housing 2 in the outside of the hollow fiber membranes. A seal 10a is provided between the housing 2 and the jacket 3 to block gas-flow passing from the inlet 17 to the outlet 18. Gas-holding parts 20a, 20b are formed by being surrounded on three sides by the housing 2, the jacket 3 and the seal 10a.