PURPOSE: To reduce and shorten the trouble and time for checking and to reduce a cost by connecting a measuring instrument capable of sending and receiving data to a connection terminal through an attenuator whose attenuation quantity is set properly and making a check according to whether or not data from a detection terminal is received correctly.

CONSTITUTION: A portable checker 8 is connected to a star coupler 3 through a semifixed attenuator 7, whose attenuation quantity is set to the maximum transmission loss permitted between a detection terminal 4 and the start coupler 3 to transmit data between both terminals; when the data transmission is performed excellently as a result, it is judged that an optical transmission line has a sufficient transmission margin in this period. For a transmission line to the center, the portable checker 8 is connected instead of a master station 2 to make a check similarly. In this case, the attenuation quantity of the semifixed attenuator 7 is set to the maximum value of the transmission margin including the detection terminal 4, start coupler 3, and an optical fiber F.