PURPOSE: To obtain a rope free from dimensional change with time and useful for set-net, etc., by passing a rope of a polyolefin strand through a pressure vessel having inlet and outlet, sealing the vessel while keeping the rope in stretched state, evacuating the vessel and heating the rope with high-pressure steam.

CONSTITUTION: A rope 1 of a polyolefin strand is passed from an inlet to an outlet through a pressure vessel A having an inlet and an outlet sealable with sealing mechanisms B. A tension is applied to the rope 1 to an extent not to slacken the rope with rope-holding mechanisms D, E placed at the outside of the inlet and outlet of the vessel A. The rope 1 is sealed, the vessel A is evacuated while keeping the rope in the above state and high-pressure steam is introduced into the vessel A with a high-pressure steam supplying means I to effect the heat-treatment of the rope 1. The sealed state is broken, the rope of the heat-treated part is moved and delivered from the vessel to introduce an untreated rope 1 into the vessel A, the rope is stopped and the transfer and heat-treatment of the rope 1 are intermittently repeated to perform the heat- treatment of the rope 1.