  In order to study photomultiplier's short-term gain stability at high
counting rate, we constructed an LED pulsed light source and its output monitor
system. For the monitor system, we employed a photon counting method using a
photomultiplier as a monitor photon detector. It is found that the method
offers a simple way to monitor outputs from a pulsed light source and that,
together with an LED light source, it provides a handy way to investigate
photomultiplier's rate effects.
