PURPOSE: To reduce a change in a secondary pressure even if a flow rate is increased by defining the shapes of a main valve and its peripheral communicating passage so that fluid flowing around the main valve does not apply axial direction force to the main valve as much as possible.

CONSTITUTION: An air stream flows along the outer diameter changing part 17c of the main valve 17, and then a part of the stream flows along an expanding/contracting passage 15c and then flows from a communicating hole 15d to a secondary pressure takeout port 13. Both the hole 15 and changing part 17c are prepared so that the area of the passage is smoothly expanded or contracted symmetrically around the center in the axial direction. Even if downward moving force is applied to the main valve 17 a Bernoulli's force generated by an air stream flowing along the lower half of the changing part 17c, upward moving force is applied to the valve 17 by Bernoulli's force due to the air stream flowing along the upper half of the changing part 17c, so that both moving forces are balanced and large axial direction force is not applied to the valve 17.