A prediction image generation method using two prediction images to generate a prediction image by a device is provided. First and second prediction image are generated. Bidirectional prediction gradient change prediction processing is performed by using a first shift value and difference values of the first and second prediction images respectively in horizontal and vertical directions to generate a first, second, third and fourth gradient images. Motion information is derived by using the first and second prediction images, the first, second, third and fourth gradient images, a second shift value, and a third shift value. Motion compensation correction value is derived by using the motion information and the first, second, third and the fourth gradient images. The prediction image is generated by using the first and second prediction images and the motion compensation correction value. The first, second and third shift values are respectively equal to 6, 4 and 1.