PURPOSE: To make concentration measurement possible by LIF(laser inductive fluorescence method) even in liquid combustion site, solid combustion site, etc.

CONSTITUTION: The first and second pigment lasers 13a and 13b are oscillated by an excitation pulse laser 11, and two laser lights of different wave lengths are outputted with a slight time lag. The two laser lights are, after combination, applied on a measurement field 18 through a beam expander. At this time, two excited to-be-measured molecule and fluorescence occurring from a background part are imaged with two CCD cameras 20a and 20b which are synchronized with the pigment lasers 13a and 13b. At that time, the CCD camera 20a images the background light and fluorescence from the to-be-measured molecule and the other camera 20b images only the background light. By subtracting the output of one CCD camera from that of the other, only the fluorescence emitted from the to-be-measured molecule is extracted, so that two dimensional concentration distribution of the to-be-measured molecule is obtained.