To improve combustibility of a lean-burn gas engine and to obtain higher engine thermal efficiency, maintaining NOx (nitrogen oxides) contained in exhaust to a lower degree.

By directing a communication hole 14 between a precombustion chamber 13 and a main combustion chamber 1 in a direction along swirl flow within the main combustion chamber 1 and inclining the direction of frame jet jetted from this communication hole 14 by a prescribed angle for a cylinder center line, combustion speed is increased by utilizing flow energy of swirl flow a combustion rate is improved and a fuel consumption rate is improved.