A high pressure tank installed in an emulsifier and connected between a bubble generator and a water pipe to receive a liquid from a water pump for enabling the liquid to be delivered to the bubble generator for making bubbles, the high pressure tank having a venturi tube extended from one side thereof to the water pipe for enabling a part of the liquid to be guided out of the high pressure tank to the water pipe for circulation, and an air valve installed in the venturi tube for enabling outside air to be drawn into the venturi tube to mix with the liquid passing through.