To provide an image composing device and an image composing method capable of performing an image composing with minimized hardware.

In the image composing device 1, an image memory reading control part 18 controls the order of image data to be read from an image memory 11 for every pixel designated by a pixel number counter 112 so as to correspond to the superposing order of image data. Image data inputted to an α-blend computing element 13 through a CLUT 12 are subjected to α-blend operation with data stored in an arithmetic processing memory 14. The output of the computing element 13 is written in the memory 14 and used as an object of α-blend operation of image data to be read next from the image memory 11. The α-blend operation result for all the superposed image data is written in an arithmetic result storage memory 15 and outputted as image composing data.