A piston adapter, which can be easily removed even when it is mounted on a piston member of a rodless syringe and pressed in a cylinder member, is provided. In piston adapter 200, when manual operation member 230 is manually rotated relative to adapter rod 210, adapter shaft 220 is also rotated to turn the trailing end of flange engagement member 240 engaging with an engagement open/close mechanism near leading end of the flange engagement member, thereby piston adapter 200 can be easily put on and removed from the piston member of the rodless syringe its leading end by manually operating the trailing end of piston adapter 200.
