A parameter for each of a plurality of images captured in time series is computed based on information obtained from the image, and a normal reference image (an image captured before an image targeted for processing is stored). A degree of similarity between the image targeted for processing and the normal reference image is computed, and a parameter to be used in image processing applied to the image targeted for processing is computed by performing weighted addition such that a parameter computed from the normal reference image has a higher weight than a parameter computed from the image targeted for processing the higher the degree of similarity.