The present invention relates to a multi-bit-rate optical communication method, optical network units, and an optical line terminal that enable multi-bit-rate transmission of low bit-rate data and high bit-rate data while maintaining compatibility with existing systems and reducing upgrade costs of optical network units and labor required for the upgrade, wherein the optical line terminal transmits a first data area including frame synchronization information in a predetermined frame format at a first bit rate and a second data area at a second bit rate which is the first bit rate×Mdown (Mdown>1), and any of the optical network units performs reception processing at a rate once every Mdown-bit to detect the frame synchronization information in the first data area and, based on detection timing thereof, performs reception processing of the second data area by a bit.