A shape determining device (X) splits the original light beam from a light source (Y) into two light beams, directs the light beams to the front and back surfaces of the object (1) to be determined, and performs optical heterodyne interference using the split light beams at the front and back surfaces of the object (1) to be determined. In the shape determining device (X), each of the split light beams is further split into a main light beam and a subordinate light beam, the subordinate light beam interferes with the main light beam at each of the front and back surfaces before and after the illumination of the object (1) to be determined, the signals after the interference are phase-detected, and the difference between the phases acquired by the phase detection is detected at each of the front and back surfaces of the object (1) to be determined. In the shape determining device (X), optical modulation for performing the optical heterodyne interference is performed before the optical heterodyne interference of the split light beams after the split light beams are directed to the front and back surfaces of the object (1) to be determined. Furthermore, in the shape determining device (X), a measurement optical system before the phase detection and after the split light beams are directed to the front and back surfaces of the object (1) to be determined is maintained integrally.