A novel, convenient method of particle size measurement by use of a photomultiplier is proposed. The measuring equipment consisted of a source of parallel light beams and an optical fiber connected to a photomultiplier which was used as a detector for the variation of light intensity. When one particle passed through the detecting section, the output of the photomultiplier changed according to the diameter of the particle. The relationship between the photomultiplier output and the particle diameter was numerically computed by using the theory of geometrical optics, and the results were confirmed experimentally. This report describes the computed results, the circuit for measurement, the method of calibrating the detectors, the estimation of errors due to the coexistence of particles in the slit, and examples of particle size measurements.