Disclosed is a method of generating a diagnostic image by using a photoacoustic material. The method includes determining a wavelength band of light maximally absorbed by a photoacoustic material, based on at least one or more photoacoustic materials injected into an object, irradiating light, which corresponds to the determined wavelength band, onto an internal region of interest (ROI) of the object to acquire a photoacoustic signal corresponding to the irradiated light, generating a photoacoustic image indicating the ROI by using the photoacoustic signal, and combining and displaying the generated photoacoustic image and an image expressing that the light is being irradiated onto the ROI.
