To extend fatigue life and facilitate releasing of a broken disk without needing any independent supporting members by forming a weak part for releasing a breakable member by a breakable member or close to it in response to a pressure fluid.

A broken disk 28 contains recessed and projecting parts 25 connected to an annular flange part 27, the recessed and projections parts 25 are placed close to an entrance supporting member 12, a projecting surface is formed in the broken disk 28 side and a corresponding recessed surface is formed in a side close to an exit supporting member 14. A weak region 30 formed by one or a plurality of material regions distorted by the partial shearing motion of the reverse bending broken disk 28 plots a substantially circular expansion part 48 and the expansion part 48 is connected to the broken disk 28 by a non-weak screw region between the opposite ends of the weak region 30. Thus, a weak portion is supported by the flange of the disk and the necessity of using independent supporting member is eliminated.