The present invention relates to a frame communication system composed of a code converter and a frame generator/decomposer. In order to improve a line efficiency without encapsulating transmitting data, a code converter in a frame transmitting device converts at least different specific codes X and Y within the transmitting data respectively into codes (Y, A) and (Y, B) composed of the code Y and at least specific codes A and B different from each other, and a frame generator prepares a frame of the transmitting data with the code X as a frame boundary. A frame decomposer in a frame receiving device recognizes a specific code X as a frame boundary to decompose receiving data, and a code converter converts codes (Y, A) and (Y, B) within the receiving data composed of a specific code Y, and specific codes A and B into codes X and Y.