PURPOSE: To make the generation of a difference in a pressure compatible with shortening of an overall length by a method wherein a number of diaphragms are arranged in juxtaposition in an impact wave delay tube, a short tube is mounted to a part of the diaphragms, and a vacuum pump is disposed so as to perform exhaust through the short tube.

CONSTITUTION: An impact wave delay tube 20 has a cylinder the both end surfaces of which are closed with side plates 20a to the central part of each of which the piping of a beam line is connected. In an internal cavity 20b, a number of sets of two different diaphragms 21 and 22 are disposed at equal intervals in an axial direction from the atmospheric pressure side toward the duct side, and the outer diameter of each diaphragm is secured to an inner peripheral surface 20c. Further, the diaphragms 21 and 22 are formed in the shape of a cone, throughports 21a and 22a are opened to the central part of the diaphragms, and a short tube 22b is mounted to the through-hole 22a of the one diaphragm 22. A number of vacuum pumps 23 are arranged in juxtaposition on an outer peripheral surface 20d, and gas in an internal space 20b and a beam line is exhausted through an exhaust port 23a.