A data transmission system in an EPON includes a parser classifying frames generated in each block, and transmitting the frames to a block. A scheduler performs multiplexing of frames, then, during a transmission time permitted by an OLT, determines a transmission sequence of the frames and transmits the frames. A MPCP slave processes messages transmitted from the OLT, generates response messages on the transmitted messages, and maintains time synchronization with the OLT A processor interface conveys frames transmitted from the OLT to a processor, frames primitives transmitted from the processor, and transmits the primitives to the OLT. A register stores a value used during generation of frames in each block, a value for system control, and a value acquired from MPCP control frames.