Microscopic bubble generating apparatus

A microscopic bubble generating apparatus includes a Roots-type pump including a pair of multi-lobed Roots rotors housed in a pump casing, and a drive motor rotating the rotors, a suction pipe connected to a suction port of the pump casing, a discharge pipe connected to a discharge port of the pump casing, and a conduit communicating with the suction pipe and having an air inlet and a collision member with which sucked water is caused to collide. The pump is operated to suck water through the suction pipe and to cause the water mixed with air sucked through the air inlet to collide with the collision member so that a number of bubbles are generated. The bubbles are refined by a compressing action of the pump to be rendered microscopic. The water containing the microscopic bubbles is released from the discharge pipe into water in a septic tank or river.