PURPOSE: To improve the control responsiveness of the bubble injection quantity by branching the air guide side of an air passage into multiple branch passages, and providing opening/closing type solenoid valves on the branch passages, respectively.

CONSTITUTION: The water in a bathtub sucked through a water feed port 12 is guided to nozzles 14-24 provided on a bathtub 10 by a circulating pump 30 and pipes 32, 34, 36, 38. Air pipes 46, 48, 50 connected to the nozzles 14-24 are branched to branch pipes 46a-40b, solenoid valves 52a-56b are provided on the branch pipes 46a-50b, and orifices 70 are provided on the branch pipes 46b, 48b, 50b. The solenoid valves 52a-56b are opened or closed to control the bubble injection quantities of the nozzles 14-24. The solenoid valves 52a-56b are opened or closed at a very high speed, they quickly respond to the action of a flow control switch, and the bubble injection quantities can be controlled. The solenoid valves 52a-56b have a simple structure, they rarely fail, and they are excellent in durability.