A method of controlling a NOx purification system configured to reduce NOx contained in an exhaust gas, the system including: an oxidation catalyst device, an ammonia-based solution supply unit, and a selective reduction type NOx catalyst device which are arranged in order from an upstream side of an exhaust gas passage, the ammonia-based solution supply unit configured to supply an ammonia-based solution to the exhaust gas passage; a bypass passage branching from the exhaust gas passage at an upstream side of the oxidation catalyst device and joining together with the exhaust gas passage at an upstream of the ammonia-based solution supply unit; and the system also including a control unit and the control unit includes a supply amount controller which is configured to control an ammonia-based solution supply unit for adjusting an amount of the ammonia-based solution to be supplied and first and second flow-rate control valves (11a, 15a) for adjusting a flow rate of the exhaust gas which flows through the bypass passage, the method is characterized by the steps of: estimating whether a volume of NO 2 adsorbed in the oxidation catalyst device increases or decreases on a basis of volumes of NOx respectively in stages before and after the oxidation catalyst device, controlling a flow rate of the exhaust gas which passes the oxidation catalyst device and a flow rate of the exhaust gas which passes the bypass passage on a basis of the increase or decrease in the estimated volume of adsorbed NO 2, wherein when the estimated volume of adsorbed NO 2 increases, the flow rate of the exhaust gas which passes the bypass passage is set to zero; when the estimated volume of adsorbed NO 2 decreases, part of the exhaust gas is caused to pass the bypass passage, and the remaining part of the exhaust gas is caused to pass the oxidation catalyst device; when the estimated volume of adsorbed NO 2 neither increases nor decreases, a proportion (molar ratio) of NO 2 in NOx contained in the exhaust gas which flows out of the oxidation catalyst device is estimated; when the estimated proportion of NO 2 therein is not larger than 50%, all of the exhaust gas is caused to pass the oxidation catalyst device; and when the estimated proportion of NO 2 therein is larger than 50%, part of the exhaust gas is caused to pass the bypass passage, and the remaining part of the exhaust gas is caused to pass the oxidation catalyst device. estimating whether a volume of NO 2 adsorbed in the oxidation catalyst device increases or decreases on a basis of volumes of NOx respectively in stages before and after the oxidation catalyst device, controlling a flow rate of the exhaust gas which passes the oxidation catalyst device and a flow rate of the exhaust gas which passes the bypass passage on a basis of the increase or decrease in the estimated volume of adsorbed NO 2, wherein when the estimated volume of adsorbed NO 2 increases, the flow rate of the exhaust gas which passes the bypass passage is set to zero; when the estimated volume of adsorbed NO 2 decreases, part of the exhaust gas is caused to pass the bypass passage, and the remaining part of the exhaust gas is caused to pass the oxidation catalyst device; when the estimated volume of adsorbed NO 2 neither increases nor decreases, a proportion (molar ratio) of NO 2 in NOx contained in the exhaust gas which flows out of the oxidation catalyst device is estimated; when the estimated proportion of NO 2 therein is not larger than 50%, all of the exhaust gas is caused to pass the oxidation catalyst device; and when the estimated proportion of NO 2 therein is larger than 50%, part of the exhaust gas is caused to pass the bypass passage, and the remaining part of the exhaust gas is caused to pass the oxidation catalyst device.