To set a space between two bearing clips for respectively connecting bearings rotatably supporting a first roller and a second roller to the predetermined dimension.

The first roller 1 an the second roller 2 are arranged in two shafts a and b, avoiding an abutment on each other. A third roller 3 and a fourth roller 4 abutting on both of the first and the second rollers are arranged at the predetermined friction angle between the first roller 1 and the second roller 2. A cylindrical spacer 32 is provided to set a space between the two bearing clips 16 and 16 respectively connecting the bearings 15 and 15, which rotatably support the first roller 1 and the second roller 2, to each other at both ends of the rollers 1 and 2 to the predetermined dimension.