A gas purifying apparatus oxidatively decomposes and discharges impurities such as flammable hydrocarbons contained in gas generated from a toner by fixing an image. A variable cursor position defined by a parameter corresponding to a position in the input register selects a set of pre-cursor and post-cursor taps for dynamic ISI correction of a like set of pre-cursor and post-cursor symbols. The controller may read the output value and compare the output value with a predetermined value.