To attach a rubber blanket even when an opening of a cylinder groove is narrow by providing insert string pieces at its front and back edges which are to be connected to each other in a clamp.

Blanket end parts 7 and 8 respectively have trapezoidal cross sections so as to be stabilized. The end parts 7 and 8 come in contact with each other at their side faces 24 and 26, and are equipped with insert string pieces 19 and 21 formed to be thicker than a thickness of a blanket 2, so-called reinforcing members. A clamp formed of a clamping axis has a receiving passage 11 extended in an axial direction for receiving a connecting rail 9 for connecting the blanket end parts 7 and 8. The blanket end parts 7 and 8 having the insert string pieces 19 and 21 are firstly pushed into the connecting rail 9 at the axial direction. Next, the blanket end parts 7 and 8 connected in such a way are inserted into the receiving passage 11 of the clamp in the axial direction together with the connecting rail 9. Thereafter, a dissociable locking member protects the axial direction of the blanket end parts 7 and 8.