Single crystal germanium (Ge) surfaces were irradiated with 100 keV Ga ions at room temperature at doses up to 3×1017 cm-2 and were observed by a scanning electron microscope (SEM) and an atomic force microscope (AFM). It is found that the Ga-irradiated regions swelled and nano-porous structures were formed on the surface after the Ga irradiation up to a dose of about 2×1017 cm-2, but the region was etched by the Ga irradiation for larger doses. The atomic composition of the swelled and etched surfaces was investigated by an Auger electron spectroscopy (AES), but no Ga signal was observed, indicating that the precipitation of Ga did not influence on these characteristics of the Ga-irradiated Ge surfaces. To investigate the water resistance of such nano-porous structures, the sample was dipped in water up to 10 hours, and the surface was observed using SEM and AFM. It is found that the swelled structures were maintained, but the etched region formed by larger-dose irradiation was partially peeled off. This indicates that the nano-porous structures can be used under water environments as a functional surface, such as filters, and deep etching can be performed by larger dose irradiations and dipping the sample in water.