PROBLEM TO BE SOLVED: To provide a method and a structure for supporting a construction girder for construction of a structure under a railroad track, which are used when the structure under the railroad track, enhancing constructibility and shortening a construction period, is constructed by a cut and cover tunneling method. SOLUTION: Characteristically, this method includes: a step of exposing a track bed 1 by removing a rail 4, a sleeper 3 and ballast 2, corresponding to a construction portion of the structure under the railroad track; a step of installing the construction girder 11, in which a plurality of support portions constituted by stacking a steel plate support 8 and a foundation steel plate 9 larger in area than the steel plate support are installed in a lower portion, on the track bed via a temporary supporting member 20; a step of supporting the support portion on the track bed by adjusting unevenness between the track bed and the support portion; and a step of continuously mounting the rail on the construction girder and the adjacent rail. COPYRIGHT: (C)2011,JPO&INPIT