To eliminate connection by a bolt or the like needing to secure specified driver operating space in order to connect a rod and a plunger in an extra- high pressure triple plunger pump with the rod and plunger coaxially arranged on the base end side and tip side and separably connected so as to make the rod replaceable.

A plunger chuck 62 and a rod chuck 82 are rotatably fitted to both sides of an adaptor 42 and provided with thread grooves 72, 92 in semicircular projecting parts 70, 90. Thread ridges 32, 38 are formed at the connecting side end parts of a plunger 18 and a rod 12. When the plunger chuck 62 and rod chuck 82 are rotated by a half circle in relation to the adaptor 42, the thread grooves 72, 92 are thread-fastened to the thread ridges 32, 38 to connect the plunger 18 and rod 12 to each other.