To make it possible to finish repairs in a short down time even if an electron accelerator is out of order by holding a means for accelerating electrons where an accelerating tube, a scanning tube and a cooling wind header constitute a set in a moving base so that they can be drawn out.

A shield plate 26 is fixed onto a moving base 30, and a connection route 23 is fixed on the plate 26. Then, an accelerating tube 22 is fixed on the moving base 30 by a stay 21 outside a shield chamber 10, a scanning tube 24 is fixed on the shield plate 26 by a stay 25 inside the shield chamber 10 and a cooling wind header 28 is located in an opening of the scanning tube 24. Moreover, a cooling wind duct 35 is buried inside the moving base 30, is exposed in a buried part 38 on the side where ventilated air exhausts and is connected to the cooling wind header 28. A cooling blower 34 is also fixed behind the accelerating tube 22 on the moving base 30. In this way, only the scanning tube 24 is included in the shield chamber 10, so that the chamber 10 can be made compact, the construction time and materials can be reduced and an electronic accelerator 20 can also be brought out quickly.