PURPOSE: To provide a liquid-tight structure allowing liquid-tightness to be enhanced with a simple method, regarding a plunger device where a packing is pressed to maintain liquid-tightness.

CONSTITUTION: A male thread section (m) is formed on the external surface of the small diameter section 3a of a plunger rod 3, and a stepped section (d) is formed at the base end of the section (m) via a run-off section (t). Also, the diameter of the stepped section (d) is determined to suit the diameter of the coupling hole of a spring retainer 12. This spring retainer 12 accurately coupled to the stepped section (d) is tightened and fixed with a piston seat receiver 11, and supports one end of a spring 17 to press a packing structure 13. Furthermore, a collar section 12b is formed around the external surface of the spring retainer 12 for covering the periphery of the spring 17.