A compression ignition of hydrogen fuel has been investigated using a conventional swirl chamber type diesel engine. It has been suggested that the hydrogen-fueled diesel combution is attainable in spite of the expected difficulty due to a high resistance to autoignition. Smooth running can be obtained if a proper amount of the fuel leakage vitiates the charge in the swirl chamber prior to the injection, otherwise ignition fails to ignite leading to an abnormal combustion with a strong knocking and to misfiring. The conditions required for a smooth burning have been studied by varying the amount and the timing of preinjections and the quantity of the fuel leakage. The test results have shown that the fuel leakage at an adequate rate keeps the hot core in the midst of the swirl chamber from being destroyed and promotes ignition of the main fuel, thus resulting in smooth running.