Adsorption states and photochemistry of methane and deuterated methane on a Pt(111) surface have been investigated by measuring temperature-programmed desorption spectra, x-ray photoelectron spectra, work function changes, and angle-resolved time-of-flight distributions of desorbed species. Methane weakly adsorbed on the Pt(111) surface at 40 K is dissociated to methyl and hydrogen fragments with laser irradiation at 193 nm. This is remarkably different from the photochemistry of methane in the gas phase where photodissociation takes place only at lambda<145 nm. While the photofragments mostly remain on the surface, some fraction of methyl desorbs with average translational energy of 0.27 eV. Photodesorption of methane is a minor channel. Desorbed methane is sharply collimated along the surface normal and shows two hyperthermal velocity components, Among the two, the faster component is attributed to associative recombination between a methyl adsorbate and a hydrogen atom produced by the photodissociation of adsorbed methane. The photochemical processes are substantially suppressed when the surface is covered with methyl adsorbate of 0.14 ML achieved by an extensive irradiation of 193-nm photons. In contrast, no photochemical reactions result from the 193-nm irradiation of methane adsorbed on a Xe/Pt(111) overlayer or from the 248-nm irradiation of methane on the bare Pt surface, These results indicate that the photochemical processes occur only for methane in close contact with substrate atoms under the irradiation of 193-nm photons. The incident angle dependence of cross sections of the photochemistry obtained with linearly polarized light indicates that direct electronic excitation of methane adsorbate plays an important role in the photochemistry of methane. The photochemistry of methane on Pt(111) at the wavelength substantially longer than that in the gas phase implies that the electronic excited state of methane is significantly mixed with substrate electronic states.