To prevent the damage of a sidewall and a bottom face of a concrete irrigation canal caused by the pressure of water in back face sediment by effectively eliminating water in the back face sediment of the concrete irrigation canal.
This concrete irrigation canal is technically assumed that the wall surface is provided with a weep hole for draining water in sediment behind a wall. A conduit pipe is connected to the back face of the weep hole, and pores are provided in suitable parts of the conduit pipe. Water in the back face sediment is guided to the canal through these pores. In this case, the diameter of the pores of the conduit pipe is set to 5-8 mm. Sand solidified by aged deterioration can thereby be eliminated to the canal through the conduit pipe, and fluidity of water in the back face sediment can be recovered.