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 unit that computes the parameter for each of the plurality of images based on information obtained from the image; a storage unit that stores, as a normal reference image, an image captured before an image targeted for processing; a similarity computation unit 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 unit that computes a parameter to be used in image processing applied to the image targeted for processing, by performing weighted addition of a parameter computed from the image targeted for processing and a parameter computed from the normal reference image, such that the parameter computed from the normal reference image has a higher weight the higher the degree of similarity, wherein the storage unit is updated such that 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.