The present invention addresses the problem of providing a method for automatically adjusting an electron beam emitted from an electron gun equipped with a photocathode to the incident axis of an electron optical system.;[Solution];An incident axis alignment method for an electron gun equipped with a photocathode,the electron gun being capable of emitting an electron beam in a first state due to the photocathode being irradiated with excitation light, andthe method including at leastan excitation light radiation step,a first excitation light irradiation position adjustment step for changing the irradiation position of the excitation light on the photocathode and adjusting the irradiation position of the excitation light, andan electron beam center detection step for detecting whether a center line of the electron beam in the first state coincides with an incident axis of an electron optical system.
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