PROBLEM TO BE SOLVED: To provide a CO gas leakage monitoring method capable of safely monitoring CO gas leakage over a wide range. SOLUTION: A monitoring object is imaged by an infrared camera 1A through an infrared filter 4A, having an infrared transmission characteristic of a wavelength zone absorbed by CO gas, and the monitoring object is imaged by an infrared camera 1B through an infrared filter 4B, having an infrared transmission characteristic of a wavelength zone that is not absorbed by the CO gas. Then, two images acquired by the infrared cameras 1A, 1B are composited by an image compositing device 2, and a composited image is displayed on a monitor television 3, thus the CO gas leakage is monitored. COPYRIGHT: (C)2012,JPO&INPIT