A non-contact measuring method for testing the flatness of a mirror or a mirror-like surface by reflection of a Light beams from a group of point light sources at plane lattice points is mentioned. Forty-nine point light sources are arranged at the lattice points obtained by dividing each of the breadth and the length of an area one square meter into six equal parts, and the light beams from these point light sources are projected onto, in the first place, a liquid surface at rest, and, in the next place, a mirror surface to be measured. The reflecting beams thus obtained are focused into an image on a screen separately provided, and by measuring the co-ordinates of the spots of light on the screen in each case, the deviation compared with the ideal geometrical plane at each reflecting point on the mirror surface is calculated. In this paper, the authors propose the measuring principle, measuring errors and experimental results of the above method, and make it clear that the flatness of the mirror surface can be obtained mechanically by this method.