  We present a deep optical spectrum of TN J0924-2201, the most distant radio
galaxy at z = 5.19, obtained with FOCAS on the Subaru Telescope. We
successfully detect, for the first time, the CIV1549 emission line from the
narrow-line region (NLR). In addition to the emission-line fluxes of Ly alpha
and CIV, we set upper limits on the NV and HeII emissions. We use these line
detections and upper limits to constrain the chemical properties of TN
J0924-2201. By comparing the observed emission-line flux ratios with
photoionization models, we infer that the carbon-to-oxygen relative abundance
is already [C/O] > -0.5 at a cosmic age of ~ 1.1 Gyr. This lower limit on [C/O]
is higher than the ratio expected at the earliest phases of the galaxy chemical
evolution, indicating that TN J0924-2201 has already experienced significant
chemical evolution at z = 5.19.
