A misalignment calculation system comprising a misalignment calculator for calculating a misalignment occurring in butt welding of end portions of a first steel pipe and a second steel pipe by using profile data measured in a circumferential direction on the first steel pipe end portion by presetting a first angle datum, and profile data measured in a circumferential direction on the second steel pipe end portion by presetting a second angle datum. The misalignment calculator calculates the misalignment amount in a state in which an angle formed between the first and second angle datums is adjusted to an input angle and in which the center of profile data showing an external surface geometry at the first steel pipe end portion is aligned with the center of the profile data showing an external surface geometry at an end portion of the second steel pipe so that misalignment can be evaluated.