The invention achieves a communication quality diagnostic unit capable of detecting an omen phenomenon of a communication trouble early in a stage prior to actual emergence of the communication trouble due to a noise, thereby issuing a warning. The communication quality diagnostic unit for diagnosing quality of communication for transmitting and receiving frames at predetermined interval time, comprising a frame-interval time detector for monitoring interval time between the frames, a frame-interval abnormality determiner for making determination as the case of diagnostic abnormality when a predetermined threshold value is exceeded by frame-interval time as detected, and a diagnostic abnormality notifier for notifying a host unit of the diagnostic abnormality as an alarm.