An automatic blood pressure monitoring system includes a device which detects the degree of saturation of oxygen in the blood at a peripheral part of the body. The detecting device generates a signal if the degree of saturation is detected to be abnormal. A control device operates a blood pressure measuring device in response to the signal indicating abnormal oxygen saturation. In a second embodiment, a trigger device periodically generates a signal to operate the blood pressure measuring device. When an abnormal oxygen saturation is detected, a signal operates the blood pressure measuring device and the triggering interval is shortened.