PURPOSE: To prevent the disappearance of radio channel in the case of prevention of double assignment, by renewing the memory content storing the usage condition of radio channels with the result of collation with the memory contents storing the state of assignment.

CONSTITUTION: The usage state of each radio channel is written in the busy/ idle table (BIT) 21. On the other hand, the double assignment provention memory (DACM) 22 for radio channel sequentially scanns the content of the memory (LSA) storing the radio channel number assigned corresponding to the subscriber and writes in the radio channel number to the word corresponded as address. The collation circuit 23 collates the contents of BIT 21 and DACM 22 every radio channel in every rewriting of LSM by the end of circulating scanning and the result of collation renews the contents of BIT 21.