A welding defect detecting device (2) is provided with: a laser displacement sensor (50) which measures the positions of the plate surface of a plate-shaped member (110) and the plate surface of a plate-shaped member (120) in an overlapping direction; a determining unit (60) which, on the basis of the positions of the plate surface of the plate-shaped member (110) and the plate surface of the plate-shaped member (120) in the overlapping direction measured by the laser displacement sensor (50), obtains the size of a gap between the plate-shaped member (110) and the plate-shaped member (120), and if the obtained gap size exceeds a threshold, determines that there is a welding defect in a welded portion; and a reporting unit (70) which reports that there is a welding defect if the determining unit (60) has determined that there is a welding defect in the welded portion.
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