1. A method of generating a diagnostic image, the method comprising: respectively determining a wavelength band of light maximally absorbed by a plurality of different photoacoustic materials, based on the plurality of different photoacoustic materials injected into an object; irradiating different lights, which correspond to the respectively determined wavelength bands, onto an internal region of interest (ROI) of the object and acquiring different photoacoustic signals corresponding to the irradiated light; generating different photoacoustic images indicating the ROI by using the different photoacoustic signals, wherein the different photoacoustic images correspond to the plurality of different photoacoustic materials respectively; and combining the generated different photoacoustic images and an image expressing that the lights are being irradiated onto the ROI and displaying the combined image.