An optical measurement named the “transmissive light extinction method” was carried out to investigate soot behavior in a diesel combustion chamber. First, the theoretical principle was discussed, and some preliminary experiments to examine the applicability of the method were performed. The method was applied successfully to the measurement of actual soot density in an experimental engine with a facility to observe the inside of the combustion chamber.However, some limitations were noticed in the method, as it was essentially a matter of local measurement. Then a simple but unique optical arrangement was made to enable simultaneous highspeed photographing of shadowgraphs and direct images of the combustion flame.As a result, a total picture of the combustion process, though it was not sufficiently quantitative yet, was obtained, including fuel spray, flame and soot behaviors. This total picture explained well the result of the local measurement of soot density by the transmissive light extinction method.