A processing device includes a low-speed storage unit that stores an image extending in a column direction and a row direction, a high-speed storage unit on which reading is executed at a higher speed than the low-speed storage unit and which stores a region corresponding to the entire image in the column direction and a predetermined storage row that is a portion in the row direction of the image, a reading unit that continuously reads the image in the column direction from the high-speed storage unit, and a determining unit that reads the image from the low-speed storage unit and writes the image to the high-speed storage unit. The determining unit determines a row number to be read next in the image based on the minimum or maximum row number in the row direction of the image, stored in the high-speed storage unit for each of targets that are to be processed and are obtained by dividing the column direction into predetermined pixels.