1. An image processing apparatus comprising a hardware processor used for implementing each of the following units: an obtaining unit configured to obtain a first image captured using a first exposure and a second image captured using a second exposure, wherein the second exposure is higher than the first exposure; a creating unit configured to create, from the first image, a third image in which regions having high lightness of the first image are extended; and a combining unit configured to perform weighted combination of respective pixels of the first image and the second image using adding coefficients which are based on the third image, wherein the adding coefficients are predetermined such that the weight of the second image is larger as a lightness of a corresponding pixel on the third image is smaller, wherein the creating unit creates, from the first image, the third image by performing reduction processing for the first image, converting the pixel value of each pixel of the first image performed the reduction processing to a pixel value having maximum lightness within a pre-set range centered around the pixel, and enlarging the converted first image, and the pre-set range includes a range decided based on a value obtained by multiplying the number of pixels that allows a deviation of a subject, which is generated between the first image and the second image obtained by the obtaining unit, by a reduction ratio used to reduce the first image.