PURPOSE: To prevent faulty products such as assemblies of different types of parts, faulty parts, etc. from flowing out into the following process.

CONSTITUTION: Besides display devices 25, a special-purpose shelf 26 with area sensors 30 for housing assembling parts, a special-purpose pocket with a photo-electric switch 32, etc. are provided in each operating station of an assembly line. Further, the area sensors 30, etc. are interlocked with line stoppers 28 through a special-purpose shelf control device 33 and line stopper control devices 34, and thus the line stoppers 28 do not drop unless the assembling parts indicated on the display devices 25 are taken out.