A transmission relaying apparatus includes a signal receiving unit receiving a signal, transmitted from a transmission path, as a received signal; an abnormality detection unit detecting whether abnormality occurs in the received signal; and a threshold setting unit setting a preset threshold to be used for detection of abnormality by the abnormality detection unit. The threshold setting unit includes a peak value detection unit detecting a peak value of the signal level of the test signal received as the received signal by the signal receiving unit, when a test signal is output from a communication diagnostic apparatus; a threshold obtaining unit obtaining the preset threshold based on the peak value detected by the peak value detection unit; and a threshold holding unit storing the preset threshold, obtained by the threshold obtaining unit, as a preset threshold to be used by the abnormality detection unit.