Abstract: An ATM user traffic control apparatus adopts an MBEA (most behind expected arrival) multiplexing mechanism for UNI (User-to-Network Interface) and adaptively controls ATM traffic streams that a plurality of traffic sources send into a UPC (Usage Parameter Control) unit of an ATM network in consideration of traffic priorities of the traffic streams. The traffic control apparatus is provided with a plurality of small leaky bucket buffers and a plurality of virtual bucket buffers corresponding to the leaky buckets, respectively. A pair of leaky and virtual buckets are provided for each traffic source. The apparatus also includes a detection unit that detects whether the cells drained from the leaky buckets are conforming cells and provides information regarding the traffic streams where there exist conforming cells.