A normal-line information generation block 30 generates normal-line information for a frame subject to detection. A data storage block 50 stores normal-line information and the like of a key frame. A motion amount detection block 40 detects a motion amount of an imaging position of the frame subject to detection relative to an imaging position of the key frame on the basis of the normal-line information of the key frame stored in the data storage block 50 and the normal-line information of the frame subject to detection generated by the normal-line information generation block 30. Even if a positional difference of a same point is small or a luminance difference occurs between a taken image of the key frame and a taken image of a current frame, a motion amount of the imaging position of the frame subject to detection relative to the imaging position of the key frame can be accurately detected on the basis of normal-line information. Therefore, an observation position can be accurately detected.