An automatic inspection system has been popular in industrial field due to high resolution, easy to use, fast cycle time etc. Among machine-vision techniques based on camera images, there were pattern matching, color matching, edge detection, etc. Most of photos used for image processing were taken when an object was stationary. If objects to be measured keep moving in front of a camera, the boundary between the objects and the background becomes unclear, which could result in fault results on vision analysis. This paper presents experimental results on measuring the length of rods which are placed on the moving conveyer belt. Varying the velocity of the belt, the distribution of pixel values near the boundary of the rod and performance of length measurement were carefully investigated.