Abstract: A supporting structure comprises a shell and a plurality of support members which are independent of each other, inserted in the shell and arranged so that the flexural rigidity is smaller in the water flow than in a perpendecular to the water flow. The supporting structure supports one place of an axial-flow bulb turbine which is to be supported at two places spaced in the water flow direction from each other. The support structure is more flexible in the water flow direction than in the perpendicular direction to the water flow, so that small bending stresses are induced in the support structure and the axial stresses induced in the support structure by the weight and the buoyancy acting on the turbine and the bending stresses induced in the support structure by axial deformation of the turbine are kept below an allowable stress of material constituting the support structure.