The present invention relates to a versatile side-on type photomultiplier comprising a structure for improving the uniformity in light receiving sensitivity. Disposed on the outer peripheral surface of a sealed envelope of this photomultiplier is a restricting member which guides light to be detected into, of the light receiving surface of a photocathode, an effective region where the light receiving sensitivity is high, thereby restraining the light to be detected from reaching the outside of the effective region.