Provided is a resource allocating method capable of ensuring QoS according to priority levels and efficiently allocating resources. A resource allocating method according to the present invention is a resource allocating method in a radio packet communication system for letting a base station allocate a necessary resource to communication with a mobile station, which has a reservation signal transmitting step wherein the mobile station transmits a reservation signal containing priority level information of communication to let the base station receive it; a reserved time length setting step wherein, based on the priority level information in the reservation signal, a longer time is set as a reserved time length for allocation of a resource, for a high priority level than for a low priority level; and a resource allocation reserving step wherein when there is a resource available for allocation in a reserved time region defined by the reserved time length set in the reserved time length setting step, from a predetermined reservation start time, the base station allocates a resource for the communication with the mobile station having transmitted the reservation signal.