PURPOSE: To produce a square pipe excellent in strength and the accuracy of product by directly roll forming a metallic plate body into a square sectional shape from the state of flat plate according to specified roll flowers.

CONSTITUTION: A belt-like metallic plate body 17 which is a base stock for a square pipe 12 is flattened with an uncoiler 1 and leveler 2 and notched holes 13 are provided in specified positions with a notching press 4 in that flat state. After that, this metallic plate body 17 with the notched holes is directly roll formed into a square sectional shape from the state of flat plate with a 20 high roll forming mill without once forming into a round shape. Then, because the metallic plate body 17 in which the notched holes 13 are preliminarily provided is directly roll formed into the square sectional shape, one roll forming mill is enough and the cost of equipment can be reduced, and also the square pipe 12 with high-accuracy notched holes 13 can be very easily manufactured, because roll forming can be executed without generating deviation in position of the notched holes 13 and buckling of material in their circumference.