A method of lining the inner surface of a conduit, comprising placing on the site a roll of hard or semi-hard plastic pipe deformed from the original shape, i.e. a generally circular pipe to a flat pipe drawing out the thermally softened pipe from the roll toward the opening of a conduit to be lined therewith, folding in two to a shape like the letter U in front of the opening, wherein the U-folded pipe is immediately set fixedly by cooling it forcedly from the outer surface of the pipe whose shape is kept retained, put into the conduit in this shape-retained state, restored to its original shape by expanding the pipe by heat and pressure from the inner side thereof. The lining is facilitated, and the quality of lining is improved.