A photomultiplier comprising: a sealed container of which the inside is kept in a vacuum state; a photocathode, placed in said sealed container, emitting photoelectrons to the inside of said sealed container in response to light having a predetermined wavelength; a dynode unit placed in said sealed container and including a plurality of stages of dynodes emitting secondary electrons in response to the photoelectrons reached from said photocathode to cascade-multiply sequentially the secondary electrons; an anode, placed in said sealed container, taking out the secondary electrons cascade-multiplied by said dynode unit as a signal; a pair of insulating support members holding unitedly said dynode unit and said anode in a state grasping said dynode unit and said anode; a focusing electrode arranged between said photocathode and said dynode unit, and having a through hole through which the photoelectrons from said photocathode pass, said focusing electrode correcting an orbit of each photoelectron emitted from said photocathode; an accelerating electrode arranged between said focusing electrode and said dynode unit, and having a through hole through which the photoelectrons reached from said photocathode via said focusing electrode pass, said accelerating electrode accelerating the photoelectrons reached from said photocathode via said focusing electrode; and a metal disk, arranged between said accelerating electrode and said dynode unit, for supporting directly a first-stage dynode included in said dynode unit, said disk being set to the same potential as that of said first-stage dynode, wherein said metal disk has a through hole through which the photoelectrons from said cathode pass, characterised in that the shortest distance from the tube axis of said sealed container to the edge of the through hole of said metal disk is 1.3 or more times the shortest distance from the tube axis of said sealed container to the end portion of a second-stage dynode included in said dynode unit. a sealed container of which the inside is kept in a vacuum state; a photocathode, placed in said sealed container, emitting photoelectrons to the inside of said sealed container in response to light having a predetermined wavelength; a dynode unit placed in said sealed container and including a plurality of stages of dynodes emitting secondary electrons in response to the photoelectrons reached from said photocathode to cascade-multiply sequentially the secondary electrons; an anode, placed in said sealed container, taking out the secondary electrons cascade-multiplied by said dynode unit as a signal; a pair of insulating support members holding unitedly said dynode unit and said anode in a state grasping said dynode unit and said anode; a focusing electrode arranged between said photocathode and said dynode unit, and having a through hole through which the photoelectrons from said photocathode pass, said focusing electrode correcting an orbit of each photoelectron emitted from said photocathode; an accelerating electrode arranged between said focusing electrode and said dynode unit, and having a through hole through which the photoelectrons reached from said photocathode via said focusing electrode pass, said accelerating electrode accelerating the photoelectrons reached from said photocathode via said focusing electrode; and a metal disk, arranged between said accelerating electrode and said dynode unit, for supporting directly a first-stage dynode included in said dynode unit, said disk being set to the same potential as that of said first-stage dynode, wherein said metal disk has a through hole through which the photoelectrons from said cathode pass, characterised in that the shortest distance from the tube axis of said sealed container to the edge of the through hole of said metal disk is 1.3 or more times the shortest distance from the tube axis of said sealed container to the end portion of a second-stage dynode included in said dynode unit.