A tandem type image forming apparatus drives an agitation paddle when the waste toner defecated from a plurality of image forming units and flowed and collected in a single waste toner containing member by way of the first collection port thereof gets to a predetermined level in order to level the height of the waste toner in the waste toner containing member. The quantity of waste toner flowed by way of the first collection port is estimated from the quantity of the toners supplied from the toner cartridges of the development apparatus.