Abstract: A gas visualizing apparatus includes an image sensor for detecting infrared light radiated or reflected from a background of a gas leak monitoring territory A, a laser source for emitting a detection infrared beam in two-dimensional form toward the gas leak monitoring territory A, the detection infrared beam having a wavelength absorbable by a gas "g" to be detected and a CRT for displaying information relating to gas leakage as a two-dimensional visible image corresponding to the gas leak monitoring territory A. The laser source includes a beam expander for converging or diverging the detection infrared beam to vary an area of the gas leak monitoring territory A. The image sensor includes a transmission and width varying device for setting the infrared light entering the image sensor to a band substantially centering on the wavelength of the detection infrared beam, and varying a width of the band, to realize an excellent visible image.