A seat back frame for a vehicle seat includes first and second side members spaced apart from each other, each of the first and second side members being made of ultra-high tensile strength steel, the first side member having a first longitudinal body of a substantially U-shape in cross-section, the first longitudinal body comprising a pair of first spaced apart
flange sections and a first main plate section interconnecting the first spaced apart
flange sections, the first side member having a first upper region and a first lower region, the second side member having a second longitudinal body of a substantially U-shape in cross-section, the second longitudinal body comprising a pair of second spaced apart
flange sections and a second main plate section interconnecting the second spaced apart flange sections, the second side member having a second upper region and a second lower region, an upper member arranged between the first and second upper regions of the first and second side members, the upper member being made of ultra-high tensile strength steel, the upper member having first and second end portions, the upper member being mounted, by spot-
welding, to the first and second upper regions of the first and second side members with the first and second end portions thereof being fitted in the first and second side members, and a lower member arranged between the first and second lower regions of the first and second side members, the lower member being made of ultra-high tensile strength steel, the lower member having third and fourth end portions, and the lower member being mounted, by spot-
welding, to the first and second lower regions of the first and second side members with the third and fourth end portions thereof being fitted in the first and second side members.