Light from a pulse light source (11) is split by an optical splitter (12) into two light beams. One of the light beams enters a detection medium (6) through a gate optical system (2) as a gate pulse used for observing fluorescence; and the other enters a substance (4) to be measured through an exciting optical system (3) as an exciting pulse used for generating fluorescence to irradiate the detection medium (6) with fluorescence emitted from the substance (4) through a fluorescent optical system (5). An instrument for measuring the lifetime of fluorescence can measure the variation with time of fluorescence, especially the lifetime of fluorescence efficiently with high time resolution by observing the fluorescent component passing through a region where variation of refractive index is induced by the nonlinear optical effect produced in the detection medium (6) by the gate pulse as a fluorescent image utilizing variation in the polarized state and observing the positional variation of the fluorescent image in correlation with the variation with time of fluorescence.
