1. An image processing apparatus comprising: decision means for deciding a similarity between a first pixel and a second pixel of an image based on a plurality of images obtained by photographing an identical subject at different times; and average means for subjecting the first pixel and the second pixel to weighted averaging on a basis of a decided result by the decision means, wherein the average means includes determination means for determining a weighting factor on a basis of the decided result, and multiplication means for multiplying values of the first pixel and the second pixel by the weighting factor, the average means multiplies the second pixel value by a large weighting factor by the multiplication means when the similarity between the first pixel and the second pixel is high, and multiplies the second pixel value by a small weighting factor when the similarity is low, and the average means subjects the weighted first pixel value and second pixel value to the weighted averaging.