Shocks are thought to be important in the dynamics and heating of the solar chromosphere. The observational determination of shock parameters, however, has been hardly done because of the difficulty of observation at a high spatial, temporal and spectral resolution, and the lack of an effective method of inferring physical parameters from spectral data. Our inversion of the spectral data of the $H{alpha}$ and Ca II 854.2 nm lines simultaneously taken from an intranetwork area, produced temporal profiles of temperature as well as line-of-sight velocities, from which we infer that three-minute chromospheric oscillations prevailing in the upper chromosphere are in fact trains of strong shocks with a strength of about two and a propagation speed of 20 km s-1 that carry a mechanical energy flux of 500 W m-2 upward. Our result supports the notion that shocks dominate the heating of the upper chromosphere, and probably the corona as well, at least in intranetwork regions of the quiet sun.