PURPOSE: To generate electricity, by mixing air bubbles into the flow of water which has a head as to a water surface and by collecting the air bubbles at a downstream place and utilizing the collected air.

CONSTITUTION: Air, which is mixed as bubbles 14 into drained water 4, is conveyed into a drain pipe 8 by the flow of the drained water 4 so that the air reaches a place under a float 12. Under the float 12, the drained water 4 is released into a drain passage and the air bubbles 14 rise in the water. In the float 12, the bubbles go out of the water into a space 13. The air flows from the space 13 to a constant pressure tank 10 through a flexible gas pipe 11. The air is supplied under constant pressure from the tank 10 to a pneumatic turbine generator 9 to produce electric power.