A transformer unit comprising: a core; a plurality of coils (5, 6, 7) arranged in a longitudinal direction which are wound around the core; one or two upper core fasteners (8, 9, 74) disposed in parallel and one or two lower core fasteners (1, 2, 75) disposed in parallel, respectively, which fix the core in the vertical direction by studs (15 - 22) fastening the upper and lower core fasteners, and a tank which accommodates the core, the coils (5, 6, 7), the upper core fasteners and the lower core fasteners wherein a plurality of steadying braces (13, 14, 30-33, 60-63, 76, 77) of a core coil assembly obtained by assembling the core and the coils (5, 6, 7) are disposed on upper sides of the upper core fasteners (8, 9) in a direction laterally perpendicular to said longitudinal direction, wherein each of the steadying braces includes a notch (131, 132, 141, 142) at an end of the steadying brace facing a projection (291, 292, 293, 294) disposed inside the tank, wherein a plurality of said projections (291, 292, 293, 294) is provided each having a shape to be fitted to a notch (131, 132, 141, 142) of the steadying braces (13, 14) so that each of the notches (131, 132, 141, 142) of the steadying braces (13, 14) are fitted to one of the plurality of projections (291, 292, 293, 294) when the core coil assembly is accommodated in the tank; wherein the upper core fasteners (8, 9, 74) and the lower core fasteners (1, 2, 75) each comprise thin and elongated U-shaped metal plates disposed so that U-shaped bottom portions of the U-shaped metal plates face each other with the core sandwiched between the U-shaped bottom portions, and wherein the steadying braces (13, 14, 30-33, 60-63, 76, 77) are disposed on the upper sides of said two upper core fasteners (8, 9) either to be each located at opposed sides on said upper sides of the core fasteners (8, 9) or to connect the upper sides of the upper core fasteners (8, 9) together across the space between the upper core fasteners (8, 9) or on the upper side of said one core fastener. a core; a plurality of coils (5, 6, 7) arranged in a longitudinal direction which are wound around the core; one or two upper core fasteners (8, 9, 74) disposed in parallel and one or two lower core fasteners (1, 2, 75) disposed in parallel, respectively, which fix the core in the vertical direction by studs (15 - 22) fastening the upper and lower core fasteners, and a tank which accommodates the core, the coils (5, 6, 7), the upper core fasteners and the lower core fasteners wherein a plurality of steadying braces (13, 14, 30-33, 60-63, 76, 77) of a core coil assembly obtained by assembling the core and the coils (5, 6, 7) are disposed on upper sides of the upper core fasteners (8, 9) in a direction laterally perpendicular to said longitudinal direction, wherein each of the steadying braces includes a notch (131, 132, 141, 142) at an end of the steadying brace facing a projection (291, 292, 293, 294) disposed inside the tank, wherein a plurality of said projections (291, 292, 293, 294) is provided each having a shape to be fitted to a notch (131, 132, 141, 142) of the steadying braces (13, 14) so that each of the notches (131, 132, 141, 142) of the steadying braces (13, 14) are fitted to one of the plurality of projections (291, 292, 293, 294) when the core coil assembly is accommodated in the tank; wherein the upper core fasteners (8, 9, 74) and the lower core fasteners (1, 2, 75) each comprise thin and elongated U-shaped metal plates disposed so that U-shaped bottom portions of the U-shaped metal plates face each other with the core sandwiched between the U-shaped bottom portions, and wherein the steadying braces (13, 14, 30-33, 60-63, 76, 77) are disposed on the upper sides of said two upper core fasteners (8, 9) either to be each located at opposed sides on said upper sides of the core fasteners (8, 9) or to connect the upper sides of the upper core fasteners (8, 9) together across the space between the upper core fasteners (8, 9) or on the upper side of said one core fastener.