Abstract: A focusing lens having a focal length f is provided at a position spaced a distance L apart from a substance to be measured in the fluid state, the relationship between L and f is controlled, so that the formula:0.95f.ltoreq.L.ltoreq.1.05fis constantly satisfied, and the light emitted when a high output pulse laser irradiates the surface of the substance to be measured is analyzed spectroscopically so that the influence of variations on the surface of the substance to be measured can be avoided and a stabilized laser emission spectroscopically analysis on-line can be conducted as the substance to be measured flows.