The terminating layer that covers the top layer of a GaN-based semiconductor having a principal surface which is either a non-polar plane or a semi-polar plane, is removed by performing an organic solvent cleaning process step, and replaced with an organic solvent cleaned layer. Next, by irradiating the semiconductor with an ultraviolet ray, the organic solvent cleaned layer is removed to form a surface-modified layer instead. By performing these process steps, the top layer of the GaN-based semiconductor becomes the surface-modified layer and an electrical polarity is given to the surface of the GaN-based semiconductor. As a result, the hydrophilicity, hydrophobicity and wettability of the GaN-based semiconductor can be controlled.