An image processing device for a projection system includes an image generation unit and a brightness acquisition unit, and the image generation unit generates data for projecting the specific image, as a first portion in the first data for display, which corresponds to the first superimposed region, generates a second portion in the first data for display, which corresponds to the first non-superimposed region based on first input image data in the data of the projection target image, which corresponds to the first non-superimposed region, and the second brightness, and generates a third portion in the second data for display, which corresponds to the second superimposed region based on second input image data in the data of the projection target image, which corresponds to the second superimposed region, and the first brightness.