1. A method for displaying an area where measurement processing is enabled in a three-dimensional visual sensor in which a space including a measurement target and a support surface of the measurement target is imaged with a plurality of stereo cameras to perform three-dimensional measurement of the measurement target using a produced stereo image, the method comprising: a first step of setting a position and an attitude of one of the plurality of stereo cameras such that a front view of the support surface of the measurement target is imaged, displaying a two-dimensional image produced by the one of the plurality of stereo camera on a monitor, accepting a manipulation assigning a coordinate system for the displayed image and a range of an effective area for the measurement processing based on the coordinate system, and accepting an input of a numerical value indicating a height range of the measurement target based on the coordinate system; a second step of producing three-dimensional information by setting a same specific coordinate corresponding to one numerical value in the height range accepted in the first step to each pixel constituting the two-dimensional image produced by the one of the plurality of stereo camera as a z-coordinate and by setting a z-coordinate based on an upper limit of the height range and a z-coordinate based on a lower limit to each of a plurality of coordinates indicating the effective area assigned by the range assigning manipulation; a third step of accepting a manipulation assigning a direction of a line of sight with respect to the three-dimensional information and producing a projection image by performing perspective transformation of the three-dimensional information from the direction assigned by the manipulation; and a fourth step of displaying the projection image on the monitor.