PRIORITY CONTROL METHOD

PURPOSE:To equalize acquisition of the using right of a channel and to evade the adverse influence due to the fixed using right of the channel, by exchanging the priorities between blocks of the same level as well as between two processing requests including a block of a basic level. CONSTITUTION:For four channels CH0-CH3, the CH0 and CH1 form a level-1 block 11 and the CH2 and CH3 form a level-1 block 12 respectively. Then the CH0 is used by a request and the priorities are exchanged between blocks 11 and 12. In addition, the CH0 and CH1 are switched with each other within the block 11. When the CH2 having the highest priority is used, the blocks 11 and 12 are switched with each other together with switch of channels carried out within the block 12. In such a way, the priorities are successively switched with use of channels of higher priorities so that the acquisition of channel using right is equalized and the adverse influences due to the fixed using right of channels can be evaded.