1. A motion measuring device comprising: tracking object image extracting means that extracts a tracking object image, which represents a tracking object, from a motion image captured by a monocular camera, on the basis of a feature of the tracking object; actual movement amount calculating means that calculates amounts of inter-frame movement of the tracking object image, as actual movement amounts; two-dimensional plane projecting means that generates on a two-dimensional plane a projected image of a three-dimensional model, which represents in three dimensions a capturing object captured by the monocular camera; estimated movement amount calculating means that calculates respective amounts of inter-frame movement of the projected image as estimated movement amounts with respect to six degrees of freedom regarding movement of the three-dimensional model, when the three-dimensional model makes movements respectively corresponding to the six degrees of freedom; and three-dimensional displacement estimating means that estimates amounts of three-dimensional motion of the tracking object respectively corresponding to the six degrees of freedom, by using a linear equation in which the actual movement amount and the estimated movement amounts are associated with each other.