To provide an electron beam accelerator and an electron accelerating method, which facilitate maintenance of an electron beam device in a short time.

The electron beam accelerator includes: a vacuum chamber 46 having an electron beam radiation window 24 and the vacuum chamber 46 is formed in a slender element; an electron generator 31 arranged in the vacuum chamber 46 for generating electrons; a high voltage connector 12 arranged inside the slender element for feeding electric power to the electron generator 31; and a high voltage insulator 28 for separating the high voltage connector 12 and the vacuum chamber 46. The electron generator 31 is surrounded by a housing. The housing is provided with at least one opening between the electron generator 31 and the electron beam radiating window in the housing. When a voltage is applied between the housing and the electron beam radiation window 24, electrons from the electron generator 31 are turned into an electron beam, and accelerated to the outside of the electron beam radiation window 24 sealed within the vacuum chamber 46.