Fuel conversion from heavy oil to natural gas is favoured as a quick remedy for GHG re-duction from industrial engines in ship propulsion and power generation fields thanks to fewer carbon contents in natural gas. In medium-speed gas engines, the ejection of torch flame from a pre-chamber is often used to promote flame propagation within a main com-bustion chamber. Although orifice specifications affect the ejection behaviour of the torch flame and the following combustion in the main chamber, the effects are difficult to fully understand. Some of the authors developed a constant-volume combustion vessel that simu-lated the combustion chamber around the top dead centre of a pre-chamber type gas engine and observed the effects of orifice specifications on the torch flame ejection and the com-bustion in the main chamber. Still, the correlation was not concluded due to the lack of physical examination. In the paper, the above measurement results were confirmed by using RANS-type CFD considering the larger scale of the target engines, and the physical back-ground of the effects of orifice specifications was successfully reproduced.