Method of purifying exhaust gas and exhaust gas purification system

An exhaust gas purification method for an exhaust gas purification system provided with an exhaust gas purification apparatus in which an oxidizing catalyst apparatus with an oxidizing catalyst and a diesel particulate filter are arranged in order from an upstream side in an exhaust passage of an internal combustion engine of a vehicle, or an exhaust gas purification apparatus in which a diesel particulate filter with an oxidizing catalyst is arranged in an exhaust passage of an internal combustion engine of a vehicle, comprising:

• measuring a sustaining time of a low rotation speed driving condition, when detecting that a driving condition of the internal combustion engine is the low rotation speed driving condition;
• setting a predetermined time for judgment depending on a remaining deposition amount that is a difference between a particulate matter tolerable amount of the diesel particulate filter and a particulate matter deposition estimation amount right before a start of the low rotation speed driving condition;
• dividing a tolerable deposition amount by the particulate matter deposition estimation amount per unit time at the low rotation speed driving condition to arrive at a divided value;
• making the divided value a tolerable sustaining time;
• making a difference between the tolerable sustaining time and the measured sustaining time at the start to be the predetermined time for judgment;
• and
• performing a forced regeneration control of the diesel particulate filter without being affected by a pressure difference between a front and a back of the diesel particulate filter, when the measured sustaining time exceeds the predetermined time for judgment.