In the joint mechanism, when connecting a plunger to a piston, after engaging members of a plunger-side engaging region in a retracted state pass through a first engaging wall region of a piston-side engaging region, the engaging members of the plunger-side engaging region come into a projected state in a second engaging wall region and the projected state of the engaging members is fastened by a fast block, and thereby, the plunger-side engaging region engages with the piston-side engaging region.