In a premixed compression ignition diesel engine 1 with fuel being injected midway of a intake passage 15, an intake port 6 is formed with an orifice 10 which is throttled in passage cross-sectional area at least in an opening operation of an intake valve 7. This prevents adhesion of the fuel to an inner wall of the intake passage and promotes atomization of the fuel. A combustion chamber is fed with the sufficiently atomized fuel, which prevents combustion failure in the combustion chamber to attain reduction of particulate matters.