A reference density profile is generated in an outer circumference direction of a pipe having a reference welded portion on the basis of a reference fluoroscopic image generated from a radiation detection medium when a radiation source is disposed on a central axis of the pipe. A weld inspection density profile is generated in an outer circumference direction of a pipe having an inspection target welded portion on the basis of a weld inspection fluoroscopic image. On the basis of the reference density profile and the weld inspection density profile, density correction information is calculated. The density correction information is for correcting density irregularities in the weld inspection fluoroscopic image in the outer circumference direction of the pipe. On the basis of the density correction information, the density irregularities in the weld inspection fluoroscopic image are corrected.