An air pump has an air tank (20) for temporarily storing air from a pump unit (12) before discharging the air. The air tank has a tank body having a top wall (40) on which the pump unit is placed, and a peripheral wall (42) extending downward from the top wall. The tank body has a downward facing opening. The air tank further has a metallic bottom wall member (46) engaged with the bottom surface of the peripheral wall so as to close the opening of the tank body. Bolts are passed through the metallic bottom wall member and into a metallic part of the pump unit and tightened to connect and secure the pump unit and the air tank to each other.