In a mobile communication system for carrying out packet transmission/reception between a mobile terminal having a power saving mode to reduce a power consumption and a radio base station, a packet type of a packet is judged, and a sleep timer value indicating a time duration since a last packet is transmitted/received until the power saving mode starts is set according to a judgement result. Then, at least one sleep timer for measuring an elapsed time since the packet is transmitted/received is activated, and the mobile terminal is controlled to execute the power saving mode at a timing at which the elapsed time measured by the at least one sleep timer exceeded the sleep timer value.