PURPOSE: To prevent printing fault or decrease in transfer rate caused by dew on a photosensitive roller for printing and to solve problems concerning health and hygiene or air pollution.

CONSTITUTION: Waste gas (air containing solvent gas) produced in the fixing zone in an electrophotographic printing machine 25 is sent to a heat exchanger 5, then a heater 6 and a catalyst combustor 7, where hydrocarbons which constitute the solvent are converted into carbon dioxide and water. The waste gas at a high temp. from the catalyst combustor 7 is sent to the heat exchanger 5, where the hot gas gives heat to waste gas (air containing the solvent gas) while the hot gas itself is cooled. The cooled gas is partly released through a funnel 24 to the air, while the rest of the gas is returned to the electrophotographic printing machine 25, where high humidity air and the gas is mixed to decrease the relative humidity. In this method, the humidity in the printing machine 25 is detected with injecting means 18, 19, 22 and 23 and the gas is injected while the flow amt. of the gas is controlled to maintain the constant humidity in the printing machine 25.