A bubble generating device includes a body having a front cover and a rear cover connected to the front cover. The front cover has a nozzle which has a passage defined therein. The rear cover has a space defined therein. A connection tube is connected to the rear cover and has a through hole so as to be connected with a spray gun. A bubble generating unit has a first net member and a second net member, both of which are located in the space. The space is defined into an expansion area, a pressure area and a bubble accumulating area by the first and second net members. The detergent is delivered into the expansion area and generates bubbles in the bubble accumulating area. The bubbles are accumulated into big lump of bubbles and sprayed by the air stream.