change is calculated from basis weight and machine speed before and after grade change whereby moisture percentage is kept constant during grade chagne so that sheet breaks are avoided and downtime is reduced application_number US 89979501 A description preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings fig4 is a diagrammatic view showing a calculation part for calculating the amount of change in dryer steam pressure for a grade change according to the present invention in the figure a numeral 1 indicates the calculation part of steam pressure manipulation change value consisting of a first calculation part 11 a second calculation part 12 a multiplication part for multiplying the output of the second calculation part 12 by a weighting factor and an addition part 14 for summing the outputs of the first calculation part 11 and multiplication part 13 a steam pressure prediction target value and other values are input to the first calculation part 11 where a steam pressure manipulation change value determined from a predicted value of steam pressure after grade change is calculated and output in some cases another steam pressure manipulation change value for correcting disturbances other than a change in the production amount is calculated and added to the former steam pressure manipulation change value and then the resulting value is output a basis weight target value machine speed target value and other values are input to the second calculation.