To safely and quickly assemble a horizontal beam of a steel frame by temporarily fastening web plates to obverse reverse crisscross positions of a column joint and the horizontal beam end.

The web plates 31 and 32 boring a temporarily fastening hole on the side of a large number of parallel or a large number of zigzag bolt hole groups 15a, box cross-sectional columns 10 having a joining bolt hole group and a side temporarily fastening hole bored in a joint web part, and a horizontal beam 20 having a joining bolt hole group and a side temporarily fastening hole bored in a web part near a beam end part, are prepared for joining the horizontal beam 20 to the column joint 11 of the steel frame by a high strength bolt at a job site. In a factory, temporarily fastening bolts 26 and 26 are inserted into the temporarily fastening holes, and are temporarily fastened in a state of outwardly projecting a front half of the web plates 31 and 32 from the column joint 11 and a web end part of a horizontal beam joint part. The web plates 31 and 32 are respectively temporarily fastened to the obverse reverse crisscross positions of a column beam joining part. Then, the horizontal beam 20 is lifted to the previously built-up column joining part, and after allowing the end part to abut to a joining part of the joint 11, the bolts are inserted into and fastened to a bolt hole group of the joining part by superposing the respective web plates 31 and 32, and the horizontal beam is installed.