OLT is provided with a field for storing the number of effective bits for searching ONU serial numbers, a field for storing a search start serial number and a field for storing a search end serial number, and ONU serial numbers are searched from the search start serial number to the search end serial number within the number of effective bits to automatically perform acquisition of the ONU serial number and ONU connection. An ONU automatic connection method for an ATM-PON system is provided by which an ONU serial number is searched efficiently without registering the ONU serial number from OpS to OLT.