Image processing apparatus and image processing method for estimating fixed-pattern noise attributable to image sensor

An image processing method includes the steps of: generating, based on data of a plurality of images acquired by performing imaging using a same image sensor, a weight coefficient corresponding to each image so that a weight coefficient corresponding to an image of which a brightness change is smooth is relatively large; integrating the data of the plurality of images using the weight coefficients to generate data of an integrated image; applying smoothing to the data of the integrated image to generate data of a smoothed integrated image; and generating, based on the data of the integrated image and the data of the smoothed integrated image, data of estimated fixed-pattern noise which is an estimated value of fixed-pattern noise attributable to the image sensor.