PURPOSE: To make the running speed of a magnetic tape constant, to decrease the power consumption and to prevent the elongation of a tape, by abolishing a capstan and a pinch roller and controlling the rotating speed of a reel driving motor.

CONSTITUTION: A magnetic tape is run at a prescribed speed beforehand, the numbers of revolutions n0 and n1 of supply and take-up reels are measured, the value of 1/n02+1/n12 is calculated from the result of measurements and this value is stored in a storage circuit in a tape recorder as a reference value for every type of tape cassettes. In using a tape cassette loaded to the tape recorder, numbers of revolutions n'0 and n'1 of the supply and take-up reels at running are detected and the value of 1/n'02+1/n'10 is obtained at an operation circuit. This calculated value is compared with the value of the 1/n02+1/n10 stored in the storage circuit at a comparison circuit, and the number of revolutions of a driving motor is controlled so that the difference can substantially be zero.