An auxiliary agent supply means (32, 58) is provided to supply, upstream of an exhaust gas purification means (24, 56, 78), an auxiliary agent for maintaining the exhaust gas purifying function of the exhaust gas purification means (24, 56, 78). A control means (38) for controlling the auxiliary agent supply means (32, 58), thereby regulating the amount of the auxiliary agent supplied includes a standard supply quantity determination section (40, 50, 68, 86) for determining standard supply quantity of the auxiliary agent required to maintain the exhaust gas purifying function, a target supply quantity determination section (42, 52, 70, 88) for determining target supply quantity of the auxiliary agent, by correcting the standard supply quantity on the basis of exhaust pressure, and a supply control section (44, 54, 72, 90) for controlling the auxiliary agent supply means to supply the auxiliary agent in the target supply quantity.