Provided is an underdrain excavation apparatus by which, while an underdrain is excavated in the ground, a hydrophobic material and/or a water-holding material (functional material) can be filled in the underdrain executed in a smooth and reliable manner. In the underdrain excavation apparatus, a soil cutting shank 11 which is vertically held and a bullet-like underdrain borer 12 which is provided on a lower end of the soil cutting shank 11 are pulled in a horizontal direction by a bulldozer 14 to excavate an underdrain 22 in a ground 21, and a functional material supplied via a functional material transport line 13 provided in the soil cutting shank 11 and in the underdrain borer 12 is filled in the underdrain. A gas flow generator generating a gas flow by jetting a high-pressure gas supplied from a high-pressure gas generator into a tubular body is connected with the functional material transport line 13.