PURPOSE: To attain a measurement without cutting and developing a magnetic tape by providing a peak detecting device of the same number as a magnetic transducer and an arithmetic unit to calculate a recording locus based on the generating timing of the maximum value detected by the peak detecting device.

CONSTITUTION: The signal recorded on a magnetic tape is read by magnetic transducers 8aW8n and the maximum value of the signal is detected by peak detecting devices 9aW9n. An arithmetic unit 10 obtains the shape of the recording locus from the generating timing of respective maximum values detected by the peak detecting devices 9aW9n by the calculation, outputs it from an output terminal 11 to the external part, connects an XY plotter to an output terminal 11 and displays the recording locus as a graph. A (dislocation) (e) of a point A on a control line C of the recording locus and a central line R on a logic when the recording locus is a straight line can be obtained from the counting. Thus, without cutting the magnetic tape, measurement can be executed and the development and skill are not necessary.