An ATM (asynchronous transfer mode) exchange has a buffer for system #0 and a buffer for system #1 providing P-MEM's for storing data including an A bit and for a VCI/VPI currently in use is set to "1'. Upon detecting a switchover between the systems, every B bit is reset to "0". When the exchange while changing from a master to a slave receives a cell having a slave indication and while changing from a slave to a master it receives a cell having a master indication, respective systems rewrite B bits for VCI/VPI's of respective cells to "1". In the meantime, each of the P-MEM's calculates exclusive "OR" operations between the A bit and the B bit for every VCI/VPI, and further obtains a disjunction among all the exclusive "OR" operations thus obtained. Then, when both P-MEM's obtain "0" for their respective disjunctions, cells are read out from one of the buffers in the system changing from a slave to a master. This enables the ATM exchange having a switch duplexed by mutually asynchronous active and backup systems to switchover the systems without causing a cell to be duplicated or lost.