PROBLEM TO BE SOLVED: To provide a syringe and a cylinder holder both of which are hard to damage even when a liquid of a high viscosity is injected, and a medical fluid injection system using them.SOLUTION: The syringe is mounted on the cylinder holder, which holds the flange of the syringe by a cylinder holder groove to fix the syringe, to be used as an injection device. The cylinder holder is provided with a positioning mechanism capable of being fitted in a recessed part 51 of the syringe and the syringe has the recessed part 51 fitted with the positioning mechanism.
