1. A diagnosis image generation apparatus comprising: a low resolution image signal synthesis unit configured to synthesize each of a plurality of low resolution image signals by using a signal reflected from a subject; a weight calculation unit configured to calculate a weight, which is used to synthesize a high resolution image signal, with a signal value which corresponds to each of positions of some pixels among a plurality of signal values corresponding to respective positions of a plurality of pixels forming each of the synthesized low resolution image signals; and a high resolution image signal synthesis unit configured to synthesize the high resolution image signal by applying the calculated weight to each of the low resolution image signals.