A cathode operating temperature adjusting method includes acquiring an approximate equation approximating a correlation between an emission current value in an electron beam source using a cathode and an operating temperature of the cathode at which a bias voltage becomes saturated at the emission current, measuring a current density of an electron beam from the cathode when in the state where an n-th emission current value and an n-th cathode operating temperature are set in the electron beam source, determining whether the measured current density is within a first tolerance range, changing the n-th emission current value to an (n+1)th emission current value when the measured current density is not within the first tolerance range, calculating an operating temperature of the cathode corresponding to the (n+1)th emission current value by the approximate equation, and setting the calculated operating temperature, as an (n+1)th cathode operating temperature, in the electron beam source.