To make it possible to predict the occurrence of bulking from present operation conditions and operation history information based on the knowledge that the circumstances of activated sludge are established on the histories thereof.

The operation management system includes a data base (48) for storing the operation conditions and operation histories of one or ≥2 activated sludge plants (2, 201, 202 to 20N) where the bulking occurs and the processing thereof, information processing means (a host computer 46, computers 42, 421, 422 to 42N) for retrieving the data base relating to the operation conditions and operation histories of the one or ≥2 activated sludge plants and for deciding whether the retrieved operation conditions and operation histories comply with the operation conditions and operation histories relating to the occurrence of the bulking or not and the processing thereof, and an information presenting means (a data presenting section 50) for presenting the results of the decision of the information processing means, and the processing thereof.