A developing device comprising: a development roller which bears a toner and which is opposite a photoconductor drum; a magnetic roller which is arranged opposite the development roller and which performs, with a magnetic brush, supply of the toner to the development roller and removal of the toner from the development roller; a transformer which is configured to generate an alternating-current voltage applied to the development roller; a switching portion which is configured to pass and interrupt a current to the transformer; characterized by further comprising an output control portion configured to change the duty ratio stepwise a plurality of times from the first duty ratio to the second duty ratio, said duty ratio being changed from a first duty ratio to a second duty ratio being a duty ratio of switching operations performed by the switching portion; and a capacitor in which one end is connected to the transformer and the other end is connected to the switching portion wherein the output control portion is configured to: a. make a duty ratio in a development performance mode in which an electrostatic latent image formed on the photoconductor drum is developed differ from a duty ratio in a development nonperformance mode in which the electrostatic latent image formed on the photoconductor drum is not developed, such that the duty ratio in the development performance mode is higher than the duty ratio in the development nonperformance mode, b. change the development performance mode to the development nonperformance mode between sheets in continuous printing of sheets to create an appropriately thin layer of toner on a surface of the development roller, and thereafter the mode is returned to the development performance mode, c. set the duty ratio in the development performance mode at the second duty ratio and the duty ratio in the development nonperformance mode at the first duty ratio so as to change the duty ratio, and to d. change the duty ratio from the first duty ratio to the second duty ratio at divided widths in which no magnetic saturation is produced in the transformer while a predetermined time period of a few milliseconds is being acquired. a development roller which bears a toner and which is opposite a photoconductor drum; a magnetic roller which is arranged opposite the development roller and which performs, with a magnetic brush, supply of the toner to the development roller and removal of the toner from the development roller; a transformer which is configured to generate an alternating-current voltage applied to the development roller; a switching portion which is configured to pass and interrupt a current to the transformer; characterized by further comprising an output control portion configured to change the duty ratio stepwise a plurality of times from the first duty ratio to the second duty ratio, said duty ratio being changed from a first duty ratio to a second duty ratio being a duty ratio of switching operations performed by the switching portion; and a capacitor in which one end is connected to the transformer and the other end is connected to the switching portion wherein the output control portion is configured to: a. make a duty ratio in a development performance mode in which an electrostatic latent image formed on the photoconductor drum is developed differ from a duty ratio in a development nonperformance mode in which the electrostatic latent image formed on the photoconductor drum is not developed, such that the duty ratio in the development performance mode is higher than the duty ratio in the development nonperformance mode, b. change the development performance mode to the development nonperformance mode between sheets in continuous printing of sheets to create an appropriately thin layer of toner on a surface of the development roller, and thereafter the mode is returned to the development performance mode, c. set the duty ratio in the development performance mode at the second duty ratio and the duty ratio in the development nonperformance mode at the first duty ratio so as to change the duty ratio, and to d. change the duty ratio from the first duty ratio to the second duty ratio at divided widths in which no magnetic saturation is produced in the transformer while a predetermined time period of a few milliseconds is being acquired. a. make a duty ratio in a development performance mode in which an electrostatic latent image formed on the photoconductor drum is developed differ from a duty ratio in a development nonperformance mode in which the electrostatic latent image formed on the photoconductor drum is not developed, such that the duty ratio in the development performance mode is higher than the duty ratio in the development nonperformance mode, b. change the development performance mode to the development nonperformance mode between sheets in continuous printing of sheets to create an appropriately thin layer of toner on a surface of the development roller, and thereafter the mode is returned to the development performance mode, c. set the duty ratio in the development performance mode at the second duty ratio and the duty ratio in the development nonperformance mode at the first duty ratio so as to change the duty ratio, and to d. change the duty ratio from the first duty ratio to the second duty ratio at divided widths in which no magnetic saturation is produced in the transformer while a predetermined time period of a few milliseconds is being acquired.