For the application of high-surface-area nanoporous platinum (Pt) to catalytic device, electrodes and sensors, dealloying technique, which can synthesize nanoporous Pt, was combined with surface alloying technique. As a result, nanoporous structure with ligament and pore sizes below 10 nm was successfully fabricated on the Pt plate surface. Cyclic voltammetry in H2SO4 indicated that the nanoporous structure increases the true surface area by 170 times. The approximation by spherical pore model suggested that the nanoporous surface layer has a thickness of 200 nm.