During measurements of hydrogen atoms in discharge plasmas in silane and methane, using a two-photon excitation from the ground state to the n=3 states at 205 nm, the laser-induced dissociation of molecules was found to be the dominant process of hydrogen production, rather than that due to the discharge. Although the laser intensity was kept below the threshold of atomic hydrogen production from parent gases, some reaction products were at high density which were more easily dissociated by the laser irradiation. Quantitative discussions were made of the laser-induced dissociation, and the estimates of reaction product densities were thus made.