To reduce burden of adjusting a clamping force, to improve clamping of tubes, and to reduce the production cost by setting cross-sectional widths in such that the bottom wall is thicker than the sidewall, the sidewall is thinner than the top wall, and the top wall is thicker than the bottom wall.

Thickness of a sidewall 58 is 1.1 to 2.3 times that of a non- deformed (nominal) soft plastic tube wall. Thickness of top walls 51, 52 is 2.5 to 4.5 times that of the non-deformed soft plastic tube wall. Thickness of a bottom wall 60 is 0.4 to 1.0 times that of the top walls 51, 52. Thickness of the sidewall 58 is 0.3 to 0.8 times that of the top walls 51, 52. A clamp housing 50 has a ratio of weight to length of less than 0.48g/cm, 2.28 to 3.18 mm in inside diameter, 3.0 to 6 mm in outside diameter, 65 to 90 in Shore D hardness, and has a clamping strength enough to completely close a PV tube having non-shrinkable wall of 0.5 to 1.0 mm in width.