Means for reducing the particulate matter (PM) from swirl chamber type diesel engines were searched out, and the reducing mechanisms were examined using an optically accessible engine. The following points were clarified in this study. 1.At light load, the suppression of the initial injection rate reduces PM, because SOF is reduced by the change in ignition point and smoke is reduced by the retarded flowout of the dense soot from the swirl chamber 2.Under medium and high load conditions, the main cause of the exhaust smoke is fierce spray-wall impingement which leads to fuel adhesion on the wall and the stagnation of a rich fuel-air mixture. 3.Enlarging swirl chamber volume ratio suppresses the formation of dense soot in the swirl chamber. In the main chamber, however, the soot oxidization becomes insufficient due to the mixing effect reduced by the essentially decreased chamber depth. 4.The most effective method for obtaining the sufficient soot-air mixing effect in the main chamber is to maintain the necessary depth of the trench area, which prevents the widening of the flame jet from the connecting passage and consequently ensures the twin-vortex formation.