We propose a 100 ks observation of the nearest ultraluminous infrared galaxy (ULIG) Arp220. The major objective is to search for evidence of heavily obscured nuclei. First detection of a Compton-thick source may be possible with the HXD's sensitivity, if a powerful nucleus is present, leading to a major discovery. The good quality XIS spectrum can be decisive for the origin of the strong Fe K line, barely detected with the previous XMM-Newton observation.