To obtain a method for judging the connecting state of communication lines by means of a communication system attaining cost down by using a transmitter-receiver capable of alternatively switching a transmission mode or a reception mode.

A check command is transmitted from a master computer 1 to N-number of slave computers 2-1 to 2-N connected with an optical coupler 9 through optical communication equipments 5, 5-1 to 5-N capable of alternatively switching the transmission mode or the reception mode through optical fibers 7. The computers 2-1 to 2-N judges the state of a line to the computer 1, based on the check command and transmits an answer signal. The computer 1, communicates data between with the computers 2-1 to 2-N only for prescribed period. Then the computer 1 repeats polling consisting of the process like this and judges the state of the line to the computers 2-1 to 2-N, based on the polling result of at least one time.