PURPOSE: To produce a thick bend steel pipe capable of obtaining high strength and high toughness even if internal and external face one pass welding by submarge arc welding is executed.

CONSTITUTION: A steel having a compsn. contg., by weight, 0.04 to 0.10% C, 0.20 to 0.60% Ni, 0.30% Si, 1.70% Mn, ≤0.20% Ti, and the balance Fe with inevitable impurities and in which carbon equivalent Ceq is regulated to 0.35 to 0.45 is heated to 1100 to 1200°C, and rolling is started and finished at 690 to 800 C. The obtd. steel sheet is used as stock, and a straight welded steel pipe having a compsn. in which the weld metal is constituted of 0.04 to 0.08% C, 0.25% Si, 1.70% Mn, 0.03% Mo, 0.008% Ti, 0.60% Ni, 0.003% B, 0.0060% N and ≤0.04% 0, and the balance contaminated components inevitably entering in the welding and Fe is produced. The part to be bent is subjected to bending at 980 to 1050°C, is subjected to cooling hardening at 800 to 500°C at ≥8°C/min and is tempered at 500 to 600°C.