PURPOSE: To relax tension of cable remarkably and to enable laying of a long cable by laying the cable in a conduit while supplying a plenty of liquid into a conduit section in the cable laying direction.

CONSTITUTION: A tank truck 2A is arranged at a manhole A and a hose 9A is extended to the inlet 6A of a conduit 6 in order to supply a plenty of water 7 toward a manhole B. On the other hand, a tank truck 2B is arranged at the manhole B and the hose 9A is extended in order to suck the water at the outlet 6B of the manhole B, thus forming a water flow in the conduit 6 between the manholes A and B. Under this state, one end 4 of an optical cable 1 is jointed to a pull wire 3 previously passed through the conduit 6, and then the pull wire 3 is pulled by means of a winch 5 thus laying the optical cable 1 in the conduit 6 between the manholes A and B.