The number of communication channels necessary for communication with a terminal is calculated by using type and volume of data to be transmitted. When calculated number of communication channels is one, a single communication channel is allocated based on quality information of a communication line to the terminal and, when calculated number of communication channels is more than one, then more than one communication channels are allocated based on the quality information. Allocation information used for notifying of allocated communication channels is generated based on virtual channels, which are defined in advance as channel units having a combination of a plurality of consecutive communication channels, and the allocation information is transmitted to the terminal.