PURPOSE: To measure the height of reinforcement of weld continuously and automatically by fitting a slit light source and an image pickup device on a stage whose relative position to the weld line of a material to be measured is controlled automatically, and then by processing the picture signal of a weld zone.

CONSTITUTION: On a double stage enabled to move freely in a plane perpendicular to the weld line of a material to be measured, a measurement table having an image pickup device facing a weld zone perpendicularly and a light source emitting slit light slantingly is fitted, and a detector is further provided to detect the distance between the measurement table and the material to be measured. On the basis of signals from the detector and image pickup device, the position of the measurement table is controlled and a slit optical image of the weld zone is processed to find the height of excess metal. For example, the measurement table 3 having the image pickup device 21 and slit light source 16 is fitted on the double stage consisting of stages 4 and 10 for the Y axis and X axis, and on the basis of the signals from the distance detector 23 and picture processor 33, a motor 7 is driven to locate the measurement table 3 at the best position; and the slit optical image signal of the weld zone 2 is processed to find the height of reinforcement of weld.