PURPOSE: To reduce an amount of a work at an elevated place by building a grating block by using a pair of beams positioned back to back with each other independently from a work to build a body column and securely inserting the grating block among the columns of the body of a structural body at a given stage.

CONSTITUTION: The body of a structural body is built such that a main part consists of a plurality of steel frame columns 12 and steel frame beams 14 in the shape of a rectangular frame. Independently from a work to build the body, pairs of H-steel frame large beams 22 and 24 are positioned back to back with each other to form an approximate rectangular frame. A grating block 18 where grating panels are laid over is built to flange parts 26A and 26B, and further suspension rests 40 are mounted to four corners. At a stage wherein the columns 12 of the body are erected, the grating block 18 is hoisted and inserted among a plurality of the columns 12 and secured thereto. This method progresses an assembling work for the grating 18 simultaneously with the progress of a body building work.