An inflow exceeding the sewage treatment capacity by rainfall is only discharged to rivers without sewage treatment and the contamination of rivers is increased in actuality by discharging of untreated sewage. The sewage which has been temporarily saved for sewage treatment from sewage treatment plants can only be discharged into rivers as well as the sewage inside of a interceptor pipe, of which length is from a few kilometers to tens of kilometers. Accordingly, after the rain, only rainwater remains in the storage facility of sewage treatment plants as well as inside of the interceptor pipe. As describe above, there may be a contradiction that the rainwater is only handled rather than sewage because the sewage at the time of rainfall is discharged into the river and only rainwater remains inside of interceptor pipe. Accordingly, there may be a problem that microbial reaction rate becomes slow in sewage treatment process. The present invention is an overflow chamber with a new concept that lets such sewage flow into the interceptor pipe as usual and an inlet of the interceptor pipe is shut off automatically to prevent the inflow of rainwater at the time of rainfall. After the rain, the inlet of the interceptor pipe is automatically opened to solve all of the problems that are generated by the inflow of rainwater.