To provide a detector capable of accurately detecting movements of a patient by simple processing.

A detector receives input of two or more photographed images obtained by continuously photographing a subject from a camera (#1), and extracts a feature point from each of a first photographed image and a second photographed image (#2). The feature point of the first photographed image and the feature point of the second photographed image that correspond are matched to detect a moving direction for each feature point, the moving directions are classified for each corresponding feature point, and the movement of the subject is detected on the basis of the number of feature points for each moving direction, that is obtained by subtracting the number of feature points in the moving direction which is a direction opposite to the moving direction from the number of feature points for each moving direction (#3). On the basis of the detected movement of the subject, whether or not to output information associated with the movement is determined (#4).