A communication quality controller can avoid, when periodically changing noise is abruptly occurring, disconnection of a communication line used for data communication and prevent resetting of a link speed for the data communication. During the data transmission at a beforehand set link speed, a CRC error counter monitors for a predetermined period of time whether or not an error occurs in a CRC error detector at a period less than a noise period changing periodically. According to presence or absence of the error taking place during the monitoring by the counter, a periodic noise detector determines presence or absence of a periodic noise period abruptly taking place. If presence of the periodic noise is determined, disconnection of an associated communication line is avoided, and after the occurrence of the periodic noise is terminated, the data communication is conducted at the link speed.