PROBLEM TO BE SOLVED: To visualize the gas leak state even from a separated position, and to measure the distance to the leak position. SOLUTION: A region where hydrocarbons 3 leak from a gas pipe 2 or the like is irradiated with visualizing infrared rays 4 from a light emitting part 6. The visualizing infrared rays 4 include infrared rays having two wavelengths, namely, an absorption wavelength λON absorbed by the hydrocarbons and a nonabsorption wavelength λOFF. Reflected light is received by a light receiving element 30, and a concentration distribution of the hydrocarbons 3 can be acquired on a visualizing screen 9 as a leak state image 10 from comparison of the light receiving intensity by an image processing circuit 32. Distance measuring infrared rays 5 having the wavelength λL not absorbed by the hydrocarbons 3 are also irradiated from the light emitting part 6, and received by the light receiving element 30. The position where the hydrocarbon 3 concentration becomes maximum is supposed as a leak spot, and the distance to the position is measured by a distance measuring circuit 33, and can be displayed on the visualizing screen 9 as a distance display 12. COPYRIGHT: (C)2004,JPO