Spectroscopic and electrostatic probe measurements were carried out to understand the plasma feature inside and outside a 10-kW-class direct-current arc plasma jet generator with a supersonic expansion nozzle. Ammonia and a mixture of nitrogen and hydrogen were used as the working gas. The NH3 and N2+3H2 plasmas in the throat were expected to be nearly in a temperature-equilibrium condition, although the plasmas in the expansion nozzle and in the downstream plume without substrate plates were in thermodynamical nonequilibrium states. As a result, the H-atom excitation temperature and the N2 rotational excitation temperature decreased from 7000-11000 K in the throat to about 4000 K and to 1000-1500 K, respectively, on the nozzle exit at 0.1-0.2 g/s, although the NH rotational temperature did not show an axial decrease even in the nozzle. On the other hand, each temperature was almost kept a small range in the downstream plume without substrate plates under an ambient pressure of 130 Pa except for the NH rotational temperature for NH, working gas, although in the case with a titanium plate the nonequilibrium plasma came to a temperature-equilibrium one as approaching the plate.