To control the aeration and the supply of the back wash air in accordance with the initial setting even when a CPU runs away by external noises or the like, and to prevent the insufficiently purified treated water from being discharged to the outside of a system.

This air pump comprises a pump body 1, a branched channel including a channel switching device 3 for switching the channel and having a plurality of discharge ports A, B, and a control device 5 composed of a setting part for setting a present time, a channel switching time, and a channel returning time, and a control part. The control part has the CPU with a watch dog timer, a RAM in the CPU has a normal RAM area for storing the data, and a comparison RAM area for storing the comparison data, the present time is counted by the CPU, and when the watch dog timer recognizes the runaway of the CPU, the data in both areas are compared, and the operation is continued as it is when both data are agreed with each other, and the CPU is initialized and the warning (sound, lamp or the like) is generated in the case of the disagreement.