A method of manufacturing a steel pipe stiffening brace member which has a shaft constituted of a main shaft member and an auxiliary shaft member and a stiffening steel pipe obtained by connecting side edges of four flat steels surrounding the shaft to each other. In the stiffening steel pipe, inside nook portions facing a side edge of the main shaft member are formed in stiffening members 25 and 26 having a V- or L-shaped cross section by fillet welding, and outside corner portions 25b and 26b are formed in the stiffening members 25 and 26 by partial penetration welding. Thereafter, side edges of the stiffening members 25 and 26 are abutted against each other, outside corner portions 27b and 28b are temporarily assembled by partial penetration welding, and outside corner portions 25b and 26b and outside corner portions 27b and 28b are subjected to partial penetration welding.