In order to reduce particulate emissions in diesel engines, it is necessary to clarify the fundamen-tal combustion phenomena. In this study, the transient gas diffusion flame was used as a simple and basic model of diesel combustion using acetylene fuel, and the characteristics of the transient gas diffusion flame were elucidated. The measured quantities are the temperature, the relative soot concentration obtained using the two-color method, and the relative concentration of carbon dioxide obtained using the infrared luminous method. The experimental results show that the internal structure of an acetylene gas diffusion flame is composed of four regions ; a combustion promotion region in the upstream periphery, a high temperature and high-soot-concentration region in the upstream inner part, a high-temperature and low-soot-concentration region downstream, and a low-temperature and high-soot-concentration region in the flame tip. Carbon dioxide is mainly generated upstream, and subsequently becomes concentrated in the central part due to the flow.