An image processing apparatus comprising: obtaining means for obtaining a first medical image of an object and a second medical image of the object; and difference means for obtaining a difference image after the first medical image and the second medical image are registered, characterized by comprising: change means for performing processing of changing a pixel value in the difference image based on a weighted coefficient calculated using both pixel values at a corresponding pixel position p in the first medical image and the second medical image, wherein the change means is configured to calculate the weighted coefficient based on any one of a first calculation, a second calculation and a third calculation, wherein, as the first calculation, the weighted coefficient at the corresponding pixel position p, W(p) is calculated by the following equation: W p = max Pr I 1 p | θ, Pr I 2 p | θ, wherein I1(p) and I2(p) represent a pixel value at the corresponding pixel position p in the first medical image and the second medical image respectively, Pr is a likelihood, and θ represents distribution information indicating a distribution of pixel values in a region of interest, and the function max (arguments) is a function of extracting the maximum value among the arguments, wherein, as the second calculation, the weighted coefficient at the corresponding pixel position p, W(p) is calculated by the following equation: W p = Pr I 1 p, I 2 p | θ, wherein I1(p) and I2(p) represent a pixel value at the corresponding pixel position p in the first medical image and the second medical image respectively, Pr is a likelihood, and θ represents distribution information indicating a distribution of pixel values in a region of interest, and wherein, as the third calculation, the weighted coefficient is calculated as a larger pixel value at the corresponding pixel position p among the first and the second medical image. obtaining means for obtaining a first medical image of an object and a second medical image of the object; and difference means for obtaining a difference image after the first medical image and the second medical image are registered, characterized by comprising: change means for performing processing of changing a pixel value in the difference image based on a weighted coefficient calculated using both pixel values at a corresponding pixel position p in the first medical image and the second medical image, wherein the change means is configured to calculate the weighted coefficient based on any one of a first calculation, a second calculation and a third calculation, wherein, as the first calculation, the weighted coefficient at the corresponding pixel position p, W(p) is calculated by the following equation: W p = max Pr I 1 p | θ, Pr I 2 p | θ, wherein I1(p) and I2(p) represent a pixel value at the corresponding pixel position p in the first medical image and the second medical image respectively, Pr is a likelihood, and θ represents distribution information indicating a distribution of pixel values in a region of interest, and the function max (arguments) is a function of extracting the maximum value among the arguments, wherein, as the second calculation, the weighted coefficient at the corresponding pixel position p, W(p) is calculated by the following equation: W p = Pr I 1 p, I 2 p | θ, wherein I1(p) and I2(p) represent a pixel value at the corresponding pixel position p in the first medical image and the second medical image respectively, Pr is a likelihood, and θ represents distribution information indicating a distribution of pixel values in a region of interest, and wherein, as the third calculation, the weighted coefficient is calculated as a larger pixel value at the corresponding pixel position p among the first and the second medical image. change means for performing processing of changing a pixel value in the difference image based on a weighted coefficient calculated using both pixel values at a corresponding pixel position p in the first medical image and the second medical image, wherein the change means is configured to calculate the weighted coefficient based on any one of a first calculation, a second calculation and a third calculation, wherein, as the first calculation, the weighted coefficient at the corresponding pixel position p, W(p) is calculated by the following equation: W p = max Pr I 1 p | θ, Pr I 2 p | θ, wherein I1(p) and I2(p) represent a pixel value at the corresponding pixel position p in the first medical image and the second medical image respectively, Pr is a likelihood, and θ represents distribution information indicating a distribution of pixel values in a region of interest, and the function max (arguments) is a function of extracting the maximum value among the arguments, wherein, as the second calculation, the weighted coefficient at the corresponding pixel position p, W(p) is calculated by the following equation: W p = Pr I 1 p, I 2 p | θ, wherein I1(p) and I2(p) represent a pixel value at the corresponding pixel position p in the first medical image and the second medical image respectively, Pr is a likelihood, and θ represents distribution information indicating a distribution of pixel values in a region of interest, and wherein, as the third calculation, the weighted coefficient is calculated as a larger pixel value at the corresponding pixel position p among the first and the second medical image.