A coil shaft or tube to be fitted into a flexible sleeve to serve as a flexible rotation transmission shaft, containing a partially coarsely wound coil shaft formed by winding around a core a plural number of unit wires which are arranged in intimately parallel relation with each other in the fashion of a single flat metal strip, and extracting a dummy wire afterwards from a resulting tightly wound coil shaft to form coarsely wound portions where helices of certain adjacent unit wires spaced apart in the axial direction and in certain portions of the coil shaft.
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