A thickness measuring apparatus and a thickness measuring method. The thickness measuring apparatus includes a light source outputting an extended monochromatic light with coherence; a collimating lens converting output light of the light source into incident beam of parallel ray; a beam splitter reflecting and providing the incident beam to a measurement target and transmitting first reflection light reflected on a top surface of the measurement target and second reflection light reflected on a bottom surface of the measurement target; an imaging lens disposed between the measurement target and the beam splitter with a predetermined focal distance to receive and provide the incident beam to a measurement position of the measurement target disposed on the focal distance; a camera photographing an interference fringe formed by the first and second reflection lights and outputting an interference fringe image; and a processing part.