In a normal state before active/standby switching between an active system and a standby system, in an AAL2 cell assembling/disassembling processor 11 as the standby system, an ATM cell transmission/reception control portion 6 sends ATM cells to an AAL2 cell disassembling portion 8 and an AAL2 cell assembling portion 9. Therefore, in the standby system also, AAL2pf assembling buffer data and cell-to-cell stretch handover data are continuously held. When a switching start request is supplied from a host unit 1 and active/standby switching is carried out, a sequence number alone is transferred from the active system to the standby system.