A motion image processing method and device for authenticating a user using a specific device, using motion information of an object. Time series monochrome images, obtained by photographing an object, using a camera, are input. An object is detected from an initial frame of the input time series images, using a basic shape characteristic, and a plurality of reference points to be tracked are automatically determined in the object. Then, corresponding points of the respective reference points are detected in an image frame other than the initial frame among the input time series images. Subsequently, motion information of a finger is calculated, based on the result of tracking the respective reference points and an assumption of limited motion in a 3D plane. Based on the calculated motion parameter, a solid object is subjected to coordinate conversion, and displayed if necessary. As a result of the tracking, a reference point in each frame is updated. Tracking, motion parameter calculation, and displaying are repeated with respect to subsequent frames.