Provided is a complementary apparatus for measuring a level of urea water includes an ultrasonic measurement unit installed inside a urea water tank to measure a level of urea water using an ultrasonic wave, an inductance measurement unit including a winding coil wound in a vertical direction inside the urea water tank and having a winding density varied toward one side, and measuring a level of urea water using inductance measured as a current is applied to the winding coil, a temperature measurement unit installed in the urea water tank and measuring a temperature, and a controller determining whether to operate the ultrasonic measurement unit or the inductance measurement unit according to the temperature measured by the temperature measurement unit or selectively outputting a level of urea water measured by the ultrasonic measurement unit and the inductance measurement unit.