according to this invention , a film ribbon cassette of a serial printer has a device for preventing reverse rotation of a driving roller for traversing the film ribbon , said device is adapted so as to rotate the driving roller only in the unidirectional direction . namely , according to this invention , the film ribbon cassette of the serial printer comprises a driving roller being driven by traverse operation of a carriage of the serial printer , a driving pulley directly connected to the end portion of said driving roller , a rubber belt for transmitting the rotating motion of said driving pulley and a take - up spool for winding the film ribbon which is driven by said rubber belt , said driving roller being provided with a device for preventing the reverse rotation . next , this invention will be described with reference to the accompanying drawings . fig1 shows a cross - eyed view of the film ribbon cassette according to one embodiment of this invention . according to the embodiment as shown in fig1 the film ribbon cassette 1 comprises a film ribbon 2 on which ink is coated , a driving roller mechanism 5 for traversing the film ribbon 2 in constant speed , a driving pulley 6 directly connected with the driving roller , a winding pulley 4 which is rotated by driving pulley 6 and on which the film ribbon 2 is wound , and a rubber belt for transmitting the rotating power of the driving pulley to the take - up spool 4 . thus constructed ink ribbon cassette 1 is fitted in the serial printer , the driving roller mechanism 5 is rotated by utilizing the traverse operation of the carriage of a printer body ( not shown ) and through a ribbon feed mechanism , and the film ribbon is traversed or fed in such a manner that a virgin portion of the film ribbon always lies in front of a printing head . the used portion of the film ribbon which was printed by the printing head is wound on the take - up spool 4 through the driving pulley 6 directly connected with the driving roller 5 and the rubber belt 3 , and as a result , the outer diameter of the film ribbon wound on the winding pulley 4 becomes larger as the used portion of the film ribbon increases in quantity . thus , if smooth sliding does not occur between the rubber belt 3 and the winding pulley 4 , difference in tension between a tight side and a slack side of the rubber belt 3 becomes larger . when the traverse operation of the film ribbon stops , only the difference in tension of the rubber belt 3 is applied to the driving roller mechanism 5 . therefore , the driving roller mechanism 5 is urged to rotate opposite to the normal rotating direction . fig2 shows an enlarged sectional view of a detail of the driving roller mechanism 5 . as shown in fig2 an interior of the driving roller 5 is provided with a tooth portion 7 for preventing reverse rotation on the inner periphery of its outer casing and correspondingly a ratchet wheel having a ratchet portion 8 on the end portion thereof is fixed to a spindle portion , and thus the above opposite rotation of the driving roller mechanism 5 is prevented by engaging the ratchet portion 8 with the tooth portion 7 . as shown in fig3 and 4 , a virgin film ribbon 2 is wound around a supply spool ( not shown ) to form a new ribbon roll 21 . the film ribbon 2 passes from the roll 21 through a roller 22 for back tension to a printing window 24 and is used for printing therein . the used ink ribbon 2 passes through a sponge 25 for a stopper and a spindle 26 and then between a driving roller 27 and a rider roller 28 ridden on the driving roller 27 by a spring 29 . thereafter , the film ribbon 2 passes around a spindle 30 and forms a used ribbon roll 31 on a take - up spool 4 . the new ribbon roll 21 and the used ribbon roll 31 are stacked with a separate plate interposed therebetween and are coaxial . driving means ( not shown ) of the film ribbon provided on a printing device rotate the driving roller 27 in a counterclockwise direction . the rotating power is also transmitted from the driving pulley 6 through the rubber belt 3 to the take - up spool 4 and rotates the used ribbon roll 31 in the same counterclockwise direction ( see fig4 ). tension in the film ribbon 2 occurred by the driving roller 27 and the rider roller 28 is used in order to take out the film ribbon 2 from the new ribbon roll 21 . under the state of the weaker tension , however , a toothed wheel 35 which rotates in harmony with the new ribbon roll 21 is locked by a ratchet portion 34 provided on the roller 22 for back tension and thus travel of the film ribbon 2 is not made . however , in the case that the tension became strong to some degree , driving power of the roller 22 for back tension is beyond the power of a spring 33 and thus the roller 22 is counterclockwise rotated centering around a spindle 36 . as a result , the ratchet portion 34 leaves the toothed wheel 35 and the new ribbon roll 21 is rotated , whereby the delivery of the film ribbon is started . due the above mechanism , the tension in the film ribbon 2 can be maintained constant or to an enough value to take out the ribbon from the new ribbon roll 21 . the driving roller 27 is provided with a ratchet portion 41 which is outside engaged with a tooth portion 42 for preventing reverse rotation , which is fixed to a case 1 , whereby the driving roller is adapted not so as to rotate in a reverse direction or in a clockwise direction . thus , the driving roller 27 is not reversely rotated even by the tension of the rubber belt 3 and as a result slackening does not occur in the film ribbon 2 and the film ribbon 2 is not reversely rotated .