1. An image processing device comprising: a main memory to store therein a parameter used in image processing; a parameter register to temporarily store therein the parameter; an image processor to execute the image processing on image data by using the parameter stored in the parameter register; and parameter transfer circuitry to transfer the parameter stored in the main memory to the parameter register during a period in which the image processing is not executed, wherein the image processor executes the image processing on each piece of image data of a plurality of paths, the image processing device includes a plurality of parameter registers to temporarily store therein parameters that are used in the image processing and that respectively correspond to pieces of the image data of the plurality of paths, the parameter transfer circuitry transfers, during a period in which the image processing on image data of a target path is not executed, a parameter corresponding to the image data of the target path from the main memory to an associated parameter register, the image processing device includes a plurality of start registers to store transfer instructions that are given from a processor and that respectively correspond to the pieces of the image data of the plurality of paths, the parameter transfer circuitry, when a transfer instruction is asserted to any of the plurality of start registers, transfers a parameter corresponding to image data of a corresponding path from the main memory to the associated parameter register, the image processing device includes a gate order storage to store therein an order in which gate signals are negated, the gate signals each representing an execution period of the image processing on the image data of each of the plurality of paths, and the parameter transfer circuitry determines a path corresponding to the parameter to be transferred by prioritizing the order in which the gate signals are negated.