A detection apparatus, a detection method and an electron beam irradiation apparatus for detecting deflection electrons in order to precisely focus even if a vacuum seal valve is provided. An electron beam irradiation apparatus including a vacuum seal valve mechanism which is provided in a static pressure floating pad, and opens/closes an electron beam passage with a piston so as to switch between an electron beam irradiation state and a vacuum seal state, and a deflection electron detector which is provided between the vacuum seal valve mechanism and a master disk and detects a deflection electron signal generated from the master disk with electron beam irradiation.