A gas purifying apparatus oxidatively decomposes and discharges impurities such as flammable hydrocarbons (C.sub.mH.sub.2n) contained in gas generated from a toner by fixing an image. An image forming apparatus having a gas purifying apparatus, and a method of purifying gas of the image forming apparatus are also provided. The gas purifying apparatus comprises a fan unit for ventilating air, a heating unit for heating the air ventilated by the fan unit, a catalytic unit for facilitating oxidative decomposition of impurities contained in the air, and an ozone generation unit located upstream of the heating unit and the catalytic unit in the air flow direction. By providing the ozone generation unit for facilitating the oxidative decomposition reaction by the catalytic unit for impurities contained in gas generated by fixing an image, the gas purifying rate can be improved and by minimizing the reaction area of catalyst oxidizing impurities, and reducing the manufacturing costs.