A fine bubble diffusing tube that can generate fine bubbles evenly and uniformly even when the diffusing tube has a long length, a fine bubble diffusing device using such a tube and a submerged membrane separation apparatus are produced. The fine bubble diffusing tube is provided with a cylindrical supporting tube 1, and an elastic sheet 2 with fine slits formed therein, with the elastic sheet 2 being placed to cover the outer circumference of the supporting tube 1, so that, upon supplying a gas to a space between the elastic sheet 2 and the supporting tube 1, the fine slits of the elastic sheet 2 are opened to generate fine bubbles, and the supporting tube 1 has a length L of 1000 mm or more in the longitudinal direction, with gas supply units 4 serving as gas flow inlets being attached to the two ends of the supporting tube.