The authors have proposed a solid-phase growth mechanism of tungsten oxide nanowires synthesized on sputtered tungsten films. Transmission electron microscopy observation, x-ray diffraction analyses, and some ex situ experiments were performed to verify the growth model. The nanowire nuclei are generated at irregular points on the W surface. An amorphous oxidized layer is formed on the W surface by annealing, and tungsten oxide molecules migrate on the W surface into the nuclei, thus contributing to the nanowire growth.