A patient monitoring system includes a patient monitor configured to display physiological information of a subject, and a signal relay apparatus configured to communicate with the patient monitor. The signal relay apparatus includes a sensor interface configured to receive a signal from a sensor attached to the subject, an analyzer configured to analyze the signal to acquire data corresponding to the physiological information, a communication interface configured to transmit at least one of the signal and the data to the patient monitor, and a controller configured to perform a notification when a display of the physiological information on the patient monitor meets a first abnormality condition.