A NOx sensor diagnostic device includes a NOx sensor disposed in an exhaust passage of an engine, an output value obtaining device for obtaining an output value of the NOx sensor, a spray characteristics changing device for changing spray characteristics for a specific cylinder of cylinders of the engine, and a diagnostic device for diagnosing abnormality of the NOx sensor, based on the output value of the NOx sensor that is changed as a result of the change of the spray characteristics for the specific cylinder by the spray characteristics changing device. An exhaust gas purifying system includes a NOx removal system disposed in an exhaust passage of an engine, a NOx sensor disposed in the exhaust passage at least on an upstream side of the NOx removal system in a flow direction of exhaust air flowing through the exhaust passage, and the NOx sensor diagnostic device.