PURPOSE: To prevent a window foil from being broken due to the cooling air and simplify constitution for blowing the cooling air, even when an irradiation window is formed out of a double window foil.

CONSTITUTION: An irradiation window 2 is formed out of a double window foil, with the center of a window foil 3 supported on a reinforcement bar 9 and a secondary window foil 11 laid thereunder in such state as supported on the bar 9 at the center. Also, a cooling air duct 15 is provided between the centers of both foils 3 and 11 to blow the cooling air in both directions along the surface of the foils 3 and 11. In this case, the cooling air flows from the centers of the foils 3 and 11 in both directions. Thus, each of the foils 3 and 11 is cooled and the cooling air flowing along the surface thereof comes outside through both sides of the irradiation window 2. As a result, the cooling air does not strongly collide with the foils 3 and 11.