Diagnostic device and diagnostic method

A diagnostic apparatus for an optical communication system comprising:

• a signal measurement instrument configured to be connected to a side end of a first accommodation station in a communication system, measure measurement light transmitted by a terminal connected to an end of the communication system, and measure a center wavelength and a spectrum width of a spectrum of the measurement light;
• an extraction unit configured to acquire an allowable line distance corresponding to the center wavelength and the spectrum width measured by the signal measurement instrument from an allowable line length list that is a relationship between the center wavelength and the allowable line distance that satisfies a selected spectrum width;
• a comparison unit configured to compare a transmission distance between a second accommodation station newly connected to the communication system and the terminal with the allowable line distance acquired by the extraction unit;
• and
• a determination unit configured to determine that it is not necessary to update the communication system in a case in which the transmission distance is shorter than the allowable line distance and determine that it is necessary to update the communication system in a case in which the transmission distance is longer than the allowable line distance.