PURPOSE: To raise the water collecting and draining efficiency and durability of drainage by a method in which at least water-collecting part of a drain pipe consisting of a water-collecting part and a draining part which can be freely assembled at site is covered with a netted trough-like mat having draining function.

CONSTITUTION: An underdrainage tube 2 consists of a trough-like part 5 and a connector 6 to be connected with the end of the part 5 and to make up an inlet opening 15 on the upper part of the part 5. The tube 2 is set in an excavated trench 42 and the opening 15 is covered with a mat 3 which is made up by connecting troughs 17 of a S-shaped cross section in a bendable manner and also by fixing a netted filter 36 to the upside and downside of the mat. Gravel, etc., is laid on the periphery of the tube 2 covered with the mat 3 having a draining function to form a culvert. The ground water can thus be effectively collected and drained.