1. A three-dimensional measurement method for observing a measurement object, in which one or a plurality of measurement object regions are previously defined, with an optical three-dimensional measurement unit to compute a coordinate for each measurement object region, the coordinate indicating a position relative to a predetermined reference plane facing at least the measurement object region, the three-dimensional measurement method comprising: taking an image of the measurement object to produce a two-dimensional image; setting a coordinate at a pixel corresponding to the measurement object region in the two-dimensional image, the coordinate indicating the position relative to the reference plane; setting a constant value at another pixel, the constant value being of the coordinate indicating the position relative to the reference plane; virtually disposing each of the set pixels in a three-dimensional space, the three-dimensional space being formed by two axes constituting a coordinate system of the two-dimensional image and an axis indicating the position relative to the reference plane; performing a perspective transform of each pixel from a predetermined direction; and processing and displaying a two-dimensional image produced by the perspective transform such that the measurement object region can be distinguished from another region.