A magnetic tape medium having superior dimensional stability in the width direction and off-track resistance is provided although having a very high track width in the width direction. A switchover request is received. The wheel transducer according to the invention includes a tire made of ultrasonic wave permeable materials, probes combining ultrasonic emitterreceiver, a sliding plate and the means for reducing friction between the sliding plate and the bottom of the tire of the wheel transducer while flaw detection. The method further includes shifting the absorption spectrum so that the absorption baseline approximately equals a selected absorption value in a selected absorption wavelength range.