1. A 3-dimensional data preparing method, comprising: a first step of continuously taking digital images of scenes in a surrounding area when moving from a first point, which has known position data, via a second point, which has known position data, to a third point, which has unknown position data, a second step of generating tracking points from an image acquired at said first point, and sequentially specifying said tracking points by a pursuit of said tracking points generated on said continuously acquired images, a third step of obtaining 3-dimensional data of said tracking points based on a result of orientation of said tracking points at said first point and at said second point and from position data of said first point and said second point, a fourth step of sequentially obtaining a position of said third point, based on a result of orientation of one of said tracking points acquired at said third point and based on 3-dimensional position data of said tracking point, a fifth step of obtaining 3-dimensional position data of said image including said tracking point from a result of orientation of said images including said tracking points at said first point and at said second point and from said position data at said first point and at said second point, and a sixth step of sequentially obtaining 3-dimensional position data of said images including said tracking points from a result of orientation of the images including said tracking points at said second point and at said third point and from the position data at said second point and at said third point.