A mobile communication system with a packet switching function which enables sharing of radio resources among mobile stations, wherein a mobile station which has generated a request for communication quality assurance periodically sends a packet for requesting preferential use of a radio channel in order to prevent timeout of the state transition timer, timeout of which would cancel radio channel assignment to the mobile station and bring it into a dormant state if a certain period elapses without transmission or reception of a signal, so that it can remain in the active state and hold the radio channel continuously. In addition, when the mobile station requesting communication quality assurance moves from one cell to another or requests radio channel assignment, the base station controller controls the radio base station so that the mobile station can be assigned a radio channel by sending a priority request packet.