A resistance welded steel pipe having excellent resistance to leakage under internal pressure and excellent electric resistance weld zone toughness is provided. A hot-rolled steel sheet having a composition containing, in mass %, C: 0.025 to 0.168%, Si: 0.10 to 0.30%, Mn: 0.60 to 1.90%, and one or at least two selected from Ca, Nb, V, and Ti such that Pcm is 0.20 or less is subjected to continuous cold roll forming to obtain a pipe-shaped body. In this case, tapered grooves are formed in the lateral end surface of the steel sheet such that the ratio of the tapered portions to the wall thickness of the steel sheet is 10 to 80%. Then the end surfaces of the pipe-shaped body are butted against each other and subjected to electric resistance welding to thereby obtain a pipe body. Ultrasonic waves are transmitted toward the electric resistance weld surface such that a beam width is within the range of 0.1 to 4.0 mm, and the reflected waves are used for ultrasonic flaw detection using an ultrasonic flaw detector that uses array probes to thereby inspect the soundness of the electric resistance weld zone. After or before the inspection, the electric resistance weld zone is subjected to seam annealing treatment in which the electric resistance weld zone is heated to 850 to 1,150°C and cooled at a cooling rate of 20 to 200°C/s. In the electric resistance