A detecting method and an optical apparatus using the detecting method are provided. The optical apparatus includes a structured light generation unit and a sense judging unit. When a structured light from the structured light generation unit is projected on a test surface, a test pattern and a test light spot are shown on the test surface. The sense judging unit judges whether the test surface is flat according to a deformation amount of the sensed test pattern, and acquires a distance between the test surface and the optical apparatus according to an area of the sensed test light spot. The detecting method judges whether a test surface is flat and detect a distance of the test surface. Since the structured light is used to detect the distance and the flatness of the test surface, the measuring complexity is reduced.