In applying an intermediate coating layer of thermoplastic resin onto the periphery of a central tensile member and applying a main coating of polyethylene resin, having continuous spiral grooves that are for accommodating optical fibers and are inverted periodically in direction along the length direction, onto the outer periphery of the abovementioned intermediate coating layer, a cooling medium is blown or made to flow with priority onto the grooves after melt discharge to form a spacer with which even though the minimum rib thickness of the ribs that define the abovementioned spiral grooves is 1.0 mm or less, the groove inclination angle α of the spacer cross section at the inversion parts is 18° or less.