In order to enhance the reliability of a pilot flame ignition system, three kinds of subchambers in which a pilot injector and a glow plug were set up were tested with a model combustion chamber of DISC rotary engine. A two-stroke Diesel engine's cylinder head was replaced with this model combustion chamber to simulate temporal changes of air flow and pressure fields inside the chamber as an actual engine. The behavior of the pilot flame generated in the subchamber, ignition process of main fuel spray by the pilot flame, the most suitable mixture distribution between the main chamber and the subchamber, and the effect of nozzle diameter of main injector on combustion characteristics were studied by using a high-speed video camera and ion probes. The results are the following; (1)the pilot flame reliability was improved, comparing to the type without subchmber system, especially, when the pilot injection amount was little; (2) with the same total injection amount, changing the ratio of injection amount between the pilot and the main, it was found that a properly smaller subchamber made the optimum ratio of pilot to main injection amount fall, and resulted in more active combustion. (3) the combustion was enhanced when the diameter of the main nozzle orifice was small.