zo and sp is zo u2192 h is zo e is zo and de is ns and sp is zo u2192 h is ps e is zo and de is nb and sp is zo u2192 h is pb e is zo and de is zo and sp is pb u2192 h is ps e is zo and de is zo and sp is nb u2192 h is ns______________________________________ as shown in table 1 e denotes the toner consumption deviation de denotes the toner consumption grade sp denotes the humidity and h denotes the toner supply speed deviation fig4 shows an example of calculating the toner supply speed deviation by the fuzzy inferring with the above rule 1 and rule 2 the case that the toner consumption deviation x the toner consumption grade y and the inside humidity z is taken as an example in rule 1 the degree to which input x is contained in the set pb is u03bcx according to the membership function of the toner consumption deviation the degree to which input y is contained in the set zo is u03bcy according to the membership function of the toner consumption grade and the degree to which input z is contained in the set zo is u03bcz according to the membership function of the inside humidity the minimum value is selected from u03bcx u03bcy and u03bcz a calculation is performed so as to find the minimum value of the above value and the membership function of the toner supply speed deviation and then a shaded trapezoid portion s is obtained the same calculation is also executed in rule 2 and a shaded trapezoid portion t is obtained.