A simple fluorescence decay‐time measuring system with single photon counting technique is described which uses, besides an undumped cw mode‐locked laser as an exciting source, a compact side‐on photomultiplier and IC comparators instead of a head‐on photomultiplier and a constant‐fraction discriminator. This system gives time behavior of fluorescence in the subnanosecond region or less with sufficient accuracy within a time as short as several seconds. The performance of this system is studied and factors to broaden the response time are discussed.