In order to provide a working scaffold girder structure 10 in a boiler furnace 12, a working scaffold girder carriage apparatus 100, and a method of installing a working scaffold, by which the working scaffold can be easily installed/removed in/from the boiler furnace 12, a working scaffold girder structure 10 in a boiler furnace includes: a girder body 20 that is formed by vertically stacking a pair of H-beams such that a flange of one of the H-beams faces a flange of the other H-beam and providing a truss structure 26 between the flanges; and a stringer 60 having one end supported by a side surface of the girder body 20 and the other end connectable to a side surface of a neighboring girder body 20, the stringer being foldable or unfoldable in a hinged manner with respect to the one end from the side surface of the girder body in a direction intersecting with a longitudinal direction of the girder body.