1. An image analysis apparatus comprising: a moving image input unit adapted to receive data representing a moving image of a subject irradiated with X-rays; a determination unit adapted to analyze a previous frame and a current frame of the moving image, and to determine based on the analysis result whether or not any of: a change in relative position between an exposure field of the X-rays and an observation portion of the subject, a change in imaging condition of the moving image, and a change in observation portion of the subject is detected; and a feature amount setting unit adapted to set a feature amount extracted from the current frame in the current frame when said determination unit determines that any of the changes is detected, and to set a feature amount extracted from the previous frame in the current frame when said determination unit determines that no change is detected.