To automatically and surely detect abnormality that a strip-like trimming chip continuously discharged after trimming is out of a line, in trimming an edge of a steel plate by means of a trimmer.

An edge of a steel plate 1 is continuously trimmed by a trimmer 2 arranged on the way of a manufacture line of the steel plate. A strip-like trimming chip 3 generated thereby is cut into pieces by a shearing machine 4 arranged on the trimming processing line. In such a case, abnormality may occur at the trimming chip 3' flowed through the trimming processing line such as flexure in the upper or lower directions out of the processing line. Such abnormalities are automatically sensed by photosensors 6, 6' arranged on upper and lower sides of the processing line.