An exhaust purifying device for an internal combustion engine which can restrict an influence of a measurement error in a NOx sensor provided at the downstream side of a catalyst and can optimally maintain a NOx purifying rate. The device comprises a catalytic converter (70) carrying a selective catalytic reduction catalyst provided in an exhaust passage (10) of the engine (1) to selectively reduce nitrogen oxides, a urea water adding valve (60) for adding urea water to the catalyst as a reducing agent, a NOx sensor (80) provided at the downstream side of the catalyst, and an ECU (100) for adjusting an addition amount of the urea water adding valve (60) based upon output of the NOx sensor (80), wherein a urea water addition amount adjusting process is executed under a condition that a NOx amount to be generated in the engine increases.