PURPOSE: To obtain a liquid leakage-preventing clamp device capable of forcedly mounting a flexible tube to an annular member so as to prevent liquid leakage from occurring.

CONSTITUTION: A clamp device is composed of an external retention protrusion 4 arranged at an annular connecting member 2; and an elastic clam ring A having an internal engagement protrusion 7 and an internal clamp protrusion 8 that are arranged in a mutual axial direction and can be inserted through a flexible tube 1. The ring A is formed to be able to forcedly slide along towards the tubular member 2 along a portion mounted to the tubular member 2 of the tube 1 until the engagement protrusion 7 strikes against and advances over the positioning retention protrusion 4 of the tubular member 2 and the clamp protrusion 8 is located between two adjacent annular protrusions 3 of the tubular member 2.