A weld steel pipe for high-strength line pipes which is inexpensive and has excellent low-temperature toughness; and a process for producing the pipe. A base steel plate which contains, in terms of mass%, 0.010-0.050% carbon, 0.01-0.50% silicon, 0.50-2.00% manganese, 0.0001-0.0050% sulfur, and 0.003-0.030% titanium, has an aluminum content and a molybdenum content reduced to up to 0.020 mass% and less than 0.10 mass%, respectively, has a carbon equivalent (Ceq) of 0.30-0.53 and a cracking parameter (Pcm) of 0.10-0.20, comprises up to 20% polygonal ferrite in terms of areal proportion and bainite as the remainder, and has an effective crystalgrain diameter of 20 [mu]m or smaller is formed into a pipe shape.