PURPOSE: To enhance detection accuracy and stop position accuracy, by using two speed reduction sensors and one main sensor and preliminarily reducing the moving speed of a vertically moving device at the time of the detection of a joint end by the main sensor.

CONSTITUTION: In a coating unit 3 equipped with a coking agent emitting nozzle 12, not only two speed reduction sensors 4a, 4c are provided to a vertically moving device 8 running along a joint in the running direction thereof but also a main sensor 4b is provided between both of them. When the end of the joint is detected by either one of the speed reduction sensors 4a, 4c during the running of the vertically moving device 8, the moving speed of said device 8 is reduced and, subsequently, when the end of the joint is detected by the main sensor 4b, the vertically moving deice 8 is stopped. Because the moving speed is reduced by the speed reduction sensors and the main sensor 4b reaches the end of the joint at a low speed as mentioned above, abrupt braking is applied to make it possible to stop the device 8 with good accuracy.