PURPOSE:To delete positively the state of dew condensation without stopping the drive of a tape by using a tension lever which holds a tension pin and a means which detects the positions of the tension lever and a heater. CONSTITUTION:In a steady driving mode of a magnetic tape 1, a magnet 18 fixed to the lower surface of a tension lever 17 holding a tension pin 3 is distant away from a Hall device 20. If the dew condensation is produced on the surfaces of the 1, a rotary head drum or tape guide pins 4a-4f, the tape driving load is increased to increase the back tension. Thus the lever 17 is turned against a spring 19, and the magnet 18 comes close to the device 20. Thus a magnetic field is varied. Then the state of dew condensation is solved by the signal of the magnetic field and the hot air of a heater 16. Thus it is possible to eliminate a condensation phenomenon in a short time without stopping the drive of the tape 1.