An assembling method of a stator blade ring segment, a plurality of which are assembled to form a stator blade ring including a plurality of stator blades (37,43) provided between an annular inner shroud and outer shroud, comprising the steps of: arranging a plurality of segment pieces each including one of said stator blades provided between an inner shroud portion formed by circumferentially splitting said inner shroud and an outer shroud portion formed by circumferentially splitting said outer shroud; and placing a band-shaped coupling member in a groove formed in a surface on the side opposite to a side provided with said stator blade in at least one of said inner shroud portion and said outer shroud portion, and welding said coupling member to the at least one of said inner shroud portion and said outer shroud portion on opposite sides of said groove to couple said plurality of segment pieces with said coupling member in the at least one of the inner shroud portion and the outer shroud portion, wherein said groove is formed to have a side wall portion inclined so that a width of the groove gradually decreases toward a bottom, and said coupling member is formed with protrusions on opposite sides in a width direction, when said coupling member is placed in said groove, a space is formed on the side of said bottom with respect to said protrusion and a welding groove having a substantially V-shaped section is formed on the side opposite to said bottom with respect to said protrusion between the side wall portion of said groove and a side surface of said coupling member, and welding is performed at said welding groove, the assembling method being characterised by the fact that said coupling member has a maximum width at an intermediate portion in a thickness direction. arranging a plurality of segment pieces each including one of said stator blades provided between an inner shroud portion formed by circumferentially splitting said inner shroud and an outer shroud portion formed by circumferentially splitting said outer shroud; and placing a band-shaped coupling member in a groove formed in a surface on the side opposite to a side provided with said stator blade in at least one of said inner shroud portion and said outer shroud portion, and welding said coupling member to the at least one of said inner shroud portion and said outer shroud portion on opposite sides of said groove to couple said plurality of segment pieces with said coupling member in the at least one of the inner shroud portion and the outer shroud portion, wherein said groove is formed to have a side wall portion inclined so that a width of the groove gradually decreases toward a bottom, and said coupling member is formed with protrusions on opposite sides in a width direction, when said coupling member is placed in said groove, a space is formed on the side of said bottom with respect to said protrusion and a welding groove having a substantially V-shaped section is formed on the side opposite to said bottom with respect to said protrusion between the side wall portion of said groove and a side surface of said coupling member, and welding is performed at said welding groove, the assembling method being characterised by the fact that said coupling member has a maximum width at an intermediate portion in a thickness direction.