1. A signal-processing device which processes transparent images each expressed by transparent superimposition of a target and a background different from the target, the device comprising: at least one processor configured to acquire the transparent images each including the target at a plurality of times, wherein: the at least one processor estimates a first component, which originates from the target, of the transparent image at a given time t among the plurality of times based on the transparent image at the given time t and an estimated value of a second component, which originates from the background, of the transparent image at the given time t, estimates a movement amount of the target based on the estimated first component, and updates the estimated first component and the estimated second component at the given time t based on the estimated movement amount and the acquired transparent images; a portion in each of the transparent images, at which portion the target is superimposed on the background, is a sum of the first component of the transparent image on the portion and the second component of the transparent image on the portion; and both of the first component and the second component in the portion comprise non-zero values for values representing the same image coordinates in an original image.