A TDM (Time Division Multiplexing)-based mobile communication system that switches a service type according to QoS (Quality of Service). The mobile communication system includes a base transceiver system (BTS) and a mobile station (MS) connected to the BTS. The BTS monitors whether a first service provided from the BTS to the MS is satisfied according to whether at least as much data as a predetermined transmission amount is transmitted to the MS. If the first service is dissatisfied, the BTS switches a service to a second service lower in QoS than the first service so that as much data as a transmission amount smaller than the predetermined transmission amount is transmitted to the MS.