1. A motion measuring device comprising: a processor; and a memory configured to store instructions that, when executed by the processor, cause the processor to extract 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; calculate amounts of inter-frame movement of the feature of the tracking object image, as actual movement amounts; generate 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; calculate, as estimated movement amounts, amounts of inter-frame movement of a point of the projected image which point corresponds to the feature, the amounts of inter-frame movement being calculated with respect to respective six standard movements of the three-dimensional model, when the three-dimensional model makes movements respectively corresponding to the six standard movements; and estimate amounts of three-dimensional motion of the tracking object respectively corresponding to the six standard movements, by using a linear equation in which the actual movement amount and the estimated movement amounts are associated with each other.