1. An image processing method, comprising: a recording process of recording low-resolution video data; and a resolution increasing process of increasing a resolution of the low-resolution video data recorded in the recording process, performed by a computer processor, wherein: the recording process includes: a step of obtaining, in each frame of the low-resolution video data, a high-resolution image in a window region being a portion of an entire image; and a step of learning, in each frame, a resolution conversion rule by using the high-resolution image in the window region; the resolution increasing process includes a step of increasing a resolution of the low-resolution video data by using the resolution conversion rule learned in the recording process; and in the recording process, a position of the window region is changed every frame.