of the secondary transfer roller 14 is changed in the hh high temperature and high humidity environment or in the ll low temperature and low humidity environment the appropriate voltage can be constantly applied to the secondary transfer roller 14 and defective transfer can be hence reduced thereby obtaining an excellent image incidentally although the table showing the relationship between the secondary transfer impedance z 1 and the voltage variable quantity u03b4v is used in this embodiment there are e g a method using a table showing the relationship between the secondary transfer impedance z 2 and the voltage variable quantity av and a method for making a selection from a formula having the secondary impedance z 1 or z 2 or both secondary transfer impedance z 1 and z 2 as parameters besides this further the impedance z 1 or z 2 is obtained based on the start voltage v and the detection electric current i to determine a predetermined voltage quantity u03b4v according to that impedance in the foregoing embodiment however if the start voltage v is fixed a predetermined voltage quantity u03b4v can be directly obtained from the detection electric current i by using a table or.