During treatment of positive pressure ventilator, an intubation tube is inserted into the patient’s trachea. If the intubation tube is pulled out during the treatment, the patient condition may become critical. We reported on a system which enables an alarm before the tube might be pulled out by the patient’s hand. A detection electrode in the developed system is attached to the intubation tube and we proceeded to experiment with the system. Twelve healthy volunteers participated in the experiment, and were requested to place their hand at a distance of 10 cm, 5 cm, 2.5 cm, and 0 cm (contact), respectively, from the intubation tube. The preliminary result of this experiment showed that the oscillation frequency of the developed sensor changes accordingly to the distance between the electrode and the subject’s hand. By using the result, the threshold values for the alarm could be determined. In a practicability evaluation experiment, the system made alarms successfully; an attention lamp (yellow) was lit when the subject’s hand is in less than 5 cm from the intubation tube, and a warning lamp (red) was lit when the subject’s hand is contacted with the tube.