Method of forming ply member

A method of forming a ply member by successively attaching a plurality of belt-shaped members of a constant length to an entire circumference of a forming drum and by butt-joining side edges of the belt-shaped members adjacent circumferentially of the forming drum, comprising steps of:

• forming each of the belt-shaped members to have a width equal to an integer, multiple of .pi.;
• successively attaching these belt-shaped members one by one onto a forming drum having an outer circumferential length of an integer multiple of said width and of .pi. inches so that side edges of the belt-shaped members are arranged in parallel with the axis of the forming drum;
• and
• forcing the adjacent side edges of the belt-shaped members closer together in circumferential directions of the forming drum to butt-join the faces of the side edges with each other, wherein the forming drum is a polygonal cylindrical forming drum, one side of the polygon having a length of an integer multiple of .pi.inches, and the belt-shaped members are attached to flat surfaces of the forming drum one by one.