Monitoring device, method, and program for seam welding, and storage medium

A monitoring device for seam welding, in which a band-shaped steel plate is formed into a continuous cylindrical shape by means of a roll group while the plate is transported and the two ends are hot welded together in the circumferential direction of the plate that converges to a V shape, is provided with: an image acquisition unit for capturing an image of the region including the V convergence location where both ends in the circumferential direction converge in the V shape; either a butt point detection unit for detecting the butt point or a V convergence point detection unit for detecting the V convergence point; a temperature measurement region setting unit for setting the temperature measurement region including the welded part where the welded part in the thickness of the plate starts to discharge at the surface; a brightness level calculation unit for calculating the brightness level; a temperature converter for converting to temperature; and a decision unit for determining whether the temperature of