Abstract: An Intermittently Decanted Extended Aeration (IDEA) process characterized in that an aeration period, a settling period and a decanting period are successively repeated in that order in a single reactor, and influent wastewater is transversely supplied at the bottom of the reactor and slowly diffused in the reactor while treated wastewater wich is intermittently discharged during the decanting period is provided. According to the process of the invention, wastewater can be efficiently treated without by-pass of wastewater under dry weather and wet weather conditions as well as stormy weather condition.