Provided are a thickness measuring apparatus and a thickness measuring method. The thickness measuring method includes irradiating first laser beam of a first wavelength λ1 to a transparent substrate and measuring intensity of first laser beam transmitting the transparent substrate; irradiating second laser beam of a second wavelength λ2 to the transparent substrate and measuring intensity of second laser beam transmitting the transparent substrate; and extracting a rotation angle on a Lissajous graph using the first and second laser beams transmitting the transparent substrate. A phase difference between adjacent rays by multiple internal reflection of the first laser beam and a phase difference between adjacent ray by multiple internal reflection of the second laser beam is maintained at π/2.