To provide a structure of a beam irradiation window in an electron beam accelerator that keeps guide pins from dropping without the necessity of high-level fit tolerances in work and prevents the loss of a seal effect due to the acute-angle bend of a metal ring at the edges of four corners in a beam irradiation window.

In a structure of a beam irradiation window of an electron beam accelerator, a guide pin (plunger) 13 is installed in a guide pin (plunger) installation hole 12 at the edges of four corners in a scanning tube flange 6 so that the pin 13 can freely protrude and retract. The guide pin 13 is retracted in the scanning tube flange 6 by hooking a sealing metal ring 7 on the outside of the said guide pin 13 and tightening the scanning tube flange 6 and a foil holding flange 10. In this structure, a window foil and the sealing metal ring 7 are fixed between the said scanning tube flange 6 and the foil holding flange. The guide pin 13 is installed in the scanning tube flange 6 through a hole 13a made in the center of the pin 13 with a screw 14 for preventing the pin 13 from dropping.