PURPOSE: To shorten a weld line, and to form easily a floor iron plate by forming many semi-circular notches at a prescribed interval on each opposed end of a perforated plate, and subsequently, welding a linear part between adjacent notches in each opposed and by both mutual opposed ends.

CONSTITUTION: Both perforated plates 3, 3 are provided in parallel on the same plane so that the longitudinal directions become parallel, and many semi- circular notches 6 are formed at a prescribed equal interval on each opposed end 5, 5. A linear part 7 between the adjacent notches 6 in each opposed end 5 is brought to groove working to an asymmetrical X shape. The groove side of a larger depth L is brought to preceding welding, and subsequently, back chipping is executed from the groove side of a smaller depth S, the groove side of this smaller depth S is brought to final welding, and welding of the linear part 7 is completed.