PURPOSE: To prevent a sheet from being damaged, by transmitting a power between a tension adjusting knob and a drive shaft through friction engagement so as to race the tension adjusting knob in response to an excessive turn of the tension adjusting knob caused by the operator's feeling.

CONSTITUTION: In a printing device, a sheet is fed by a tractor 3 and the like, and is printed thereon by means of a printing mechanism arranged adjacent to a platen 1. A gear drive shaft 7 driven by means of a drive gear 9 and a serration wheel 8 presses a tension adjuster knob 14 in the direction A so as to release the engagement between the gear 9 and the wheel, resulting in cut-off of the transmission of power to the platen 1. In this condition, the knob 14 is turned so as to apply a suitable tension to the sheet 4. In this case, the drive shaft 7 is attached through the intermediary of a friction disc 13 and a flange 12, and accordingly, the knob 14 is raced when it is turned excessively.