1. An image processing apparatus for computing a parameter to be used in image processing applied to each of a plurality of images captured in time series, comprising: a parameter computation processor that computes a parameter for each of the plurality of images based on information obtained from the image; a detection processor that detects a predetermined subject from an image; a storage unit that stores a subject reference image determined from at least one image in which the predetermined subject is detected and a normal reference image determined from at least one image in which the predetermined subject is not detected, out of images captured before an image targeted for processing, to which image processing is applied; a similarity computation processor that computes a degree of similarity between the image targeted for processing and the normal reference image, from a predetermined feature obtained from the respective images; and a control processor that computes a parameter to be used in image processing applied to the image targeted for processing, by performing weighted addition using (1) a parameter computed from the image targeted for processing by said parameter computation processor, (2) a parameter computed from the subject reference image by said parameter computation processor, (3) a parameter computed from the normal reference image, by said parameter computation processor, based on a result of the detection of the predetermined subject from the image targeted for processing by a detection processor and the degree of similarity, wherein the storage unit stores the image targeted for processing as a new subject reference image in a case where the predetermined subject is detected from the image targeted for processing and the degree of similarity is more than a predetermined degree of similarity, and stores the image targeted for processing serves as a new normal reference image in a case where the degree of similarity is less than or equal to a predetermined degree of similarity.