A device for real-time thickness inspection is provided. An optical interferometric technique is used. Measurement requirements in rapid online thickness inspection can be satisfied. An object is measured in a non-contact and non-destructive way. For measuring, an optical spherical wavefront is radiated on the object in an oblique angle. The interference fringe pattern (IFP) thus imaged on a screen is directly related to the thickness distribution of the object. The phase difference on the same horizontal cross section in the IFP monotonically decreases from the light source side to the other side. Accordingly, phase unwrapping can be effectively performed without using phase shift. The present invention achieves rapid on-line thickness inspection through the optical path of interference without using optical lens groups and special optical elements. The present invention radiates a high-coherence point-expanded spherical-wavefront light beam in an oblique angle for an instantaneous and wide-area full-field thickness measurement.