A system to measure the temporal behavior of atomic or molecular fluorescence, which is disturbed by large noise background, has been developed. This system uses the pseudorandom modulation technique of length up to 1023, temporal resolution of approximately 83 ns, and a photon counting capacity of 107counts/s. Data processing by a minicomputer and a hardware demodulation circuit enable quick analysis of a large volume of data. The signal‐to‐noise ratio of this measurement system is discussed with some experimental comparisons. The system is tested in the decay time measurement of I2fluorescence.