To provide a piston assembly mounting and demounting device for a control valve, allowing an application for mounting and demounting two or more types of the piston assembly, and enabling a single worker to perform the mounting and demounting work safely and easily.

This device has a plate 17 held on support bolts 15 and 16, a jack bolt 18 screwed to the plate 17 and a coupler 19 mounted on the end of the jack bolt 18. At the time of mounting and demounting a bypass piston assembly 2, a part of tightening bolts 10 is detached from screw holes 1D and, then, the support bolts 15 and 16, and a guide bolt 20 are respectively screwed to the screw holes 1D. Thereafter, the jack bolt 18 is rotated in the condition where a flange plate 9 is pressed with the coupler 19.