In a photomultiplier, a ring-like side tube is not interposed between a side tube and a stem in the radial direction, and the side tube is joined to the ring-like side tube in a state of being directly capped onto a portion of the stem that protrudes out from an open end face at the upper side of the ring-like side tube. The enlargement of the photomultiplier in the radial direction due to overlapping of the side tube and the ring-like side tube can thereby be restricted and a high density, a high degree of integration, etc., can be realized in mounting the photomultiplier.