A diffused gas aeration apparatus which is easy to be enlarged and highly efficient is provided, obtaining high performance, energy saving, space saving and maintenance free nature, by means of improvement in mixture agitation efficiency between gas and liquid. A diffused gas aeration apparatus 15 includes: a cylindrical passage tube 8 substantially placed vertically in which fluid passes in the longitudinal direction and in which a static mixer 9 is provided; a gas blowoff portion 12 on the lower end side of the passage tube 8, which supplies gas to the inside of the passage tube 8 through a pneumatic dispatch line 11 and in which a static mixer 13 is provided; wherein gas is supplied to the gas blowoff portion 12, liquid is introduced into the passage tube 8 from a liquid-introducing portion 14 on the lower side of the passage tube 8, the gas and liquid flow upward in a parallel direction in the passage tube 8 and come in gas-liquid contact inside the passage tube 8, and then are discharged from the upper end side of the passage tube 8 into liquid.