PURPOSE: To eliminate the deformation of a baffle due to its thermal shrinkage so as to easily obtain optical shielding of the baffle, by holding cones to arms applying the mothed of insertion, in the case of construction of the baffle.

CONSTITUTION: A louver-shaped baffle 18, being contained in the first stage 6 and mounted to the upper of the second stage 8 of a pump, prevents the external heat from being directly radiated to the second stage 8 further allows all gases, which can be attracted on the second stage 8 in a low temperature side, at flow through the baffle. This baffle 18 is constituted by a central plate 19, a plurality of arms 21 radially extended from the central plate to a side wall 20 of the first stage 6 of the pump and cones 27 tilted outward further downward. The arm 21 forms its internal end in a recessed shape to be fitted by a peripheral edge 22 of the central plate 19 and secured by tightening a bolt 23 and a nut 24, while the arm 21 secures its external end to a seat 26 by tightening a bolt 25.