An HO ODU signal input to an optical transmission apparatus is disassembled into LO ODUs. LO ODUs specified to form an aggregated ODU are disassembled not into individual LO ODUs but into a unit of an LO ODU group that forms an aggregated ODU. Clock extraction and an alarm process are performed not for the individual LO ODUs but for each aggregated ODU. This eliminates the need for a hardware configuration for performing the clock extraction and the alarm process in units of LO ODUs, leading to saving of hardware resources.