PURPOSE: To provide a method for producing a square pipe low in yield ratio and excellent in weatherability.

CONSTITUTION: The corners of a low allay steel square pipe contg. one or two kinds of 0.10 to 2.0% Cu and 0.070 to 0.150% P are heated to the Ac1 or above as well as to the Ac3-250°C to the Ac3-20°C, which is successively cooled to ≤200°C at ≥15°C/sec cooling rate and is tempered in the temp. range of 200 to 600°C according to necessary, by which the square pipe low in yield ratio and excellent in weatherability can be obtd. The square pipe having ≥40 kgf/mm2 high strength, low in yield ratio and excellent in weatherability can inexpensively be produced without using expensive alloy elements in particular.