Abstract: In an apparatus for determining the reduction ratio of a rolled material by measuring the moving speed of a magnetic mark formed on a strip on the inlet and outlet sides of a rolling mill, a rising of a signal of the logical product of a detection signal on the inlet side and a pulse of a gate which is opened by a magnetic mark detection signal on the outlet side and is closed after the lapse of an optimum time is adopted as the timing for detection of the magnetic mark on the inlet side, and the comparison levels of the detectors are automatically changed according to the stored magnitude of the magnetic mark, whereby a worsening of the S/N ratio of the magnetic mark signal on the inlet side or an erroneous detection is reduced.