When we produce a thermal protection system (TPS) for the space vehicle entering a planet, the arc-heated wind tunnel is used widely. The arc-heated flow is very complicated. In addition, we assume the on-board measurement of spectroscopy to understand the flow properties of very strong shock-heated gas. Present work show a method of head-on measurement of spectroscopy used optical fiber in high-enthalpy gas. At first, the head-on measurement of spectroscopy and side-on measurement were compared. At argon flow, the electronic excited temperature has 500 K difference between these two methods. At the nitrogen flow, in the case of the head-on, it could see only the spectrum from nitrogen atom. The light weight ablator (like PICA) materials was used as TPS. The head-on measurement could operate over the 10 seconds for light weight ablator. In addition, the time resolved measurement of spectroscopy caught the change of the property of spectrum.