NOVELTY - The device (10) has measurement unit (16) to measure concentration of target substance value of water, methane, carbon monoxide and carbon dioxide gases. Laser beam is made to enter in gas measurement cell into which circulation gas flows. A light-receiving unit (50) receives laser beam injected from light emission unit and passes to gas measurement cell. A calculation unit computes concentration of water, methane, carbon monoxide and carbon dioxide gases based on the intensity of laser beam output from light emission unit.    USE - Device for measuring calorific-value of gas flowing inside pipeline of combustion engine of vehicle e.g. boat and ship. Can also be used in generator and incinerator.    ADVANTAGE - Since calculation unit computes concentration of water, methane, carbon monoxide and carbon dioxide gases based on the intensity of laser beam output from light emission unit, the amount of hydrogen absorbed by measurement object can be measured with high precision and the influence of stray light by excitation light, scattered light and fluorescent lamp can be avoided. The change of carbon between circulation gases and the mass balance of hydrogen and oxygen can be measured easily. Hence the structure of the hydrocarbon can be computed at higher precision and the emitted-heat amount of circulation gas can be computed at higher precision. By providing a filter, the dust contained in circulation gas can be removed easily. The time lag generated by positioning a filter can be decreased and responsiveness can be maintained. By injecting purge gas, the foreign material adhering to window can be cleaned and generation of error during measurement can be avoided.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for method for measuring gas calorific-value.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the gas calorific-value measurement device. (Drawing includes non-English language text)    Gas calorific-value measurement device (10)    Piping unit (12)    Conversion unit (14)    Measurement unit (16)    Light-receiving unit (50)