Abstract: A non-contacting type method for measuring perpendicularity of a straight post installed perpendicularly to a reference plane is performed by picking up partial images of an upper end and a lower end of the post in two directions vertical to each other to combine obtained images into one image, calculating inclinations or deviations of four images of lower end portion and upper end portion in the combined image, and comparing the inclinations or deviations to measure the perpendicularity with respect to the reference plane. An apparatus thereof includes a measuring section for optically obtaining image data of the object to be measured, a measuring-part driving unit for driving the measuring part in the X, Y and Z directions which are perpendicular to one another, and a data processing part for processing and calculate the image data obtained by the measuring part to display the result of perpendicularity calculation.