A method of lining a channel can be provided which includes the steps of: installing at a work site a roll having wound thereon a rigid or semirigid plastic pipe shaped initially in the form of an approximately round tube and deformed from the initial form to a flat tubular form, paying off the pipe as softened by heating from the roll installed at the work site toward an inlet of the channel to be lined, and folding the paid-off flat pipe in two to a U shape at a position between the roll and the channel inlet. The U-shaped pipe is hardened by forced cooling from outside immediately after the folding step while being restricted in shape, then inserted into the channel while being thus hardened and thereafter restored to the initial form by being inflated with heat and pressure applied from inside, whereby a lining of improved quality can be formed with an improved work efficiency.