The electric conductivity of the developer is measured every constant period, a development replenisher is replenished every constant time as a time lapse replenisher in an amount based on a set value of a time lapse replenishment rate, a development replenisher is replenished every processing of a constant area of a photosensitive lithographic printing plate precursor(s) as a processing replenisher in an amount based on a set value of a processing replenishment rate, a calculated value of electric conductivity is calculated from the amounts of the time lapse replenisher and the processing replenisher, the set values of the time lapse replenishment rate and the processing replenishment rate are increased or decreased according to the result of the comparison of the calculated value with the measured value, and the value of the difference between the calculated value and the measured value is displayed as a degree of a developer activity.