PURPOSE: To facilitate assembling even in case of using steel plate, dispense with spot welding, improve working property, and reduce cost by providing a notched groove having a protruding part on each of longitudinal and transverse lattice plates, and mutually fitting and press-fitting the respective lattice plates through each notched groove.

CONSTITUTION: Mutually opposed notched grooves 6, 7 are provided on the intersecting positions of lattice plate 4, 5, respectively, the groove 6 of the plate 4 is extended from the top surface to the half depth of the lattice plate height, with a groove width almost equal to the plate thickness but not larger than it. Two inner surfaces of the groove 6 partly have protruding parts 8, 8' in the middle, respectively, and both the protruding parts 8, 8' are formed so that the space between them has a determined value slightly smaller than the plate thickness. The plate 5 is fitted to the plate 4 through the grooves 6, 7, and a force is added from above to press fit the plate 5, whereby protruding parts 10, 10' are passed through the protruding parts 8, 8', and pushed to the plate pressed side parts 16 of the plate 6 over the depth of the half, and the plates 5, 6 are mutually press-fitted and fixed. Thus, the assembling is easy in case of using steel plate, no spot welding is required to enhance working property, and cost can be reduced.