Pulsed plasma thrusters (PPTs) are electrothermal and electromagnetic thrusters that produce thrust in a discharge and have great potential as space engines owing to their simple structures. A coaxial PPT with solid propellant of cavity diameter of 3 mm and capacitor energy of 8 J has been designed and operated at Gifu University. To investigate unsteady phenomena of the PPT, optical measurements have been performed with photography using a high-speed camera. In past works, the luminescence from the PPT has been observed. The results show that the luminescence from the propellant continues after discharge. In the present study, to investigate the luminescence in detail, in addition to the photography of the luminescence without a filter, photographs were taken with a band-pass filter to obtain the luminescence from only the ionized gas, C⁺ for two different cavity lengths, 25 and 15 mm. By comparing the results between the unsteady behaviors of the luminescence with and without the filter, the luminescence phenomena were investigated. The luminescence from the propellant part without the filter in the last 34 and 15 μs for the cavity lengths of 25 and 15 mm was free of luminescence from C⁺. The time when the luminescence from C⁺ ends was close to the time of the end of discharge for each cavity length. These results suggests that the measurement of the luminescence from C⁺ is effective for understanding phenomena in the cavity during discharge.