To provide an air diffuser eliminating a space for loading and unloading the duffuser, capable of enhancing the packing efficiency of a filter medium body in an aerobic treating tank to the maximum and capable of increasing the biological treating capacity of the aerobic treating tank.

The loading and unloading of the diffuser 11 is made possible to perform without providing a space (communicating hole, etc.), for the loading and unloading the diffuser 11 in the aerobic treating tank 1 by constituting the diffuser so that the loading and unloading in the aerobic treating tank 1 is executed integrally with a filter medium body 11b by mounting integrally the filter medium body 11b to a vertical pipe 11a for supplying air from an air source to a diffusing body 11c.