amount w per printing decreases stepwise to the initial value of the initial stage at the point 1 where the cumulative number c of printed sheets is 34 u00d7 10 3 the number of sheets as shown in fig8 in the present example the boundary values of the cumulative number range cn n 1 7 of printed sheets are defined so that the web feed amount w per printing may be decreased stepwise to the initial value existent during the first stage of use when the prescribed print repetition frequency tn as the web feeding timing changes the boundary values do not always need to be set as described above but may be slightly varied the prescribed print repetition frequency tn as the web feeding timing changes for one sheet after another in the example where the width of variation allows the change may be made for every two sheets for example the same procedure for changing the web feeding timing is executed after the cumulative number range c1 of printed sheets has shifted to the second stage value c2 shown in fig7 in brief the motor 20 is repeatedly actuated to wind the web 1 every time the total of printed sheets reaches the value.