PURPOSE: To largely take the contact area of introduced gas and water and to accurately obtain saturated gas by a small-sized device by supplying water controlled in its temp. to a saturation tank storing a water retaining agent and introducing gas into the saturation tank to obtain the saturated gas.

CONSTITUTION: A water retaining material 2 such as a paper towel is received between the upper and lower water and air passable lattices 21, 22 within each of saturation tanks 11, 12. Control parts 31 controlling the temps. in the saturation tanks 11, 12 are provided to the tanks 11, 12 to also control the water temp. of the water retaining material 2. When an air feed pump 41 is operated, gas enters the saturation tanks 11, 12 from inflow ports 11a, 12a, 12b and passes through the water retaining material 2 to become saturated gas which is, in turn, discharged from outflow ports 4a, 5a. Therefore, a contact area per a unit volume can be increased and saturated gas can be accurately obtained by a small sized device.