In an optical communication system in which communication is conducted by transmitting light through a plastic optical fiber with a core diameter in the range from 300 to 600 .mu.m, the average beam diameter and beam divergence angle expressed in terms of numerical aperture (NA) of the light at the input face of the optical fiber are set less than or equal to 250 .mu.m, more preferably less than or equal to 200 .mu.m, and less than or equal to 0.25, more preferably less than or equal to 0.2 respectively.