In an air-conditioning apparatus and a method of detecting an abnormality in a transmission line, the air-conditioning apparatus includes plural facility apparatuses which are connected to each other by a transmission line and communicate with each other. One of the plural facility apparatuses includes: a transmission circuit including an abnormality detection comparator which detects an abnormality in the transmission line, the transmission circuit being provided to detect the abnormality in the transmission line; and a data storage device which stores a reception signal and signal voltage wave-height information indicating that the abnormality in the transmission line is detected. The transmission circuit further compares a voltage value of a reception signal received from another of the plural facility apparatuses through the transmission line with a threshold value set in advance in the abnormality detection comparator.