PURPOSE: To decrease tape oscillations and to decrease the friction force between an annular disk base and a magnetic tape by forming the annular disk base facing the outer peripheral surface of a rotary drum to the projecting height equal to or slightly smaller than the projecting height of the rotary head and providing spiral grooves on the outer peripheral surface of the annular disk base.

CONSTITUTION: Always the specified deformation force is applied to a magnetic tape by the annular disk base 4 so as to decrease the periodic oscillating force of the tape, by the fluctuating component in the deformation force at the time of plunging or parting of the rotary head 1 is decreased. Namely, the annular disk base 4 is projected to the outer peripheral surface of the drums 2, 3. The projecting height thereof is set equal to or slightly smaller than the projecting height of the rotary head 1, by which the magnetic tape is brought into contact with the annular disk base 4 at all times. The spiral grooves 5 are provided on the outer peripheral surface of the annular disk base 4. Then, air flows into this part. The tape oscillations are decreased in this way and the annular disk base 4 is prevented from wearing the magnetic tape.