To provide an air pump for septic tank capable of being connected even though air pipes of an aerobic pipe and a backwash pipe are laid reversely by mistake.

The air pump with a passage changeover function comprises a pump body 2, an air tank 3 for storing emitted air from the pump body 2 and discharging the air through either of discharge openings A, B, and a control device 4. The air tank 3 comprises a plurality of air chambers 11, 12, 13 partitioned by a partition wall 14, and a passage changeover device 5 installed thereon. The partition wall 14 is provided with branch ports 9, 10 for dividing an air flow passage into two. Passage changeover by the passage changeover device 5 is controlled through the control device 4. The discharge opening A is constituted of one discharge opening A1 provided at the second air chamber 12 and the discharge opening B is constituted of two discharge openings B1 and B2 provided at their common third chamber 13. The discharge opening A1 is placed between the discharge openings B1 and B2.