Patent ID: 8919102

Claim:
An exhaust purifying device for an internal combustion engine comprising: a selective catalytic reduction catalyst provided in an exhaust passage of the internal combustion engine to selectively reduce nitrogen oxides contained in an exhaust gas; reducing agent adding means for adding urea water or ammonia to the selective catalytic reduction catalyst as a reducing agent; a NOx sensor provided at the downstream side of the selective catalytic reduction catalyst in the exhaust passage; and an electronic control unit programmed to adjust an addition amount of the reducing agent based upon an output of the NOx sensor, wherein the electronic control unit is further programmed to: execute adjustment of the addition amount of the reducing agent under a condition that a NOx amount to be generated in the internal combustion engine is larger than in a normal operating time, prohibit execution of adjustment of the addition amount of the reducing agent in a case where the NOx amount to be generated in the internal combustion engine is smaller than a predetermined amount, and execute adjustment of the addition amount of the reducing agent each time when the time in which adjustment of the addition amount of the reducing agent is prohibited exceeds a predetermined threshold time.