A pre-combustion-chamber type gas engine, comprising: a pre-combustion chamber (Sr) disposed on a cylinder head portion; a spark plug disposed on an upper part of the pre-combustion chamber (Sr); a pre-combustion-chamber gas supply mechanism configured to supply ignition fuel gas to the pre-combustion chamber (Sr) via a gas supply channel for the pre-combustion chamber (Sr), the gas supply channel having an opening on an upper part of the pre-combustion chamber (Sr); and an opening-closing valve (24, 42) configured to open and close the gas supply channel for the pre-combustion chamber (Sr), wherein the opening of the gas supply channel for the pre-combustion chamber (Sr) is disposed on an upper wall of a partition wall forming the pre-combustion chamber (Sr) or on an upper section of a side wall of the partition wall, the opening facing in a tangent direction of a side-wall inner peripheral surface forming the pre-combustion chamber (Sr), wherein the ignition fuel gas supplied to the pre-combustion chamber (Sr) forms a first swirl flow which swirls about a longitudinal axis of the pre-combustion chamber (Sr) inside the pre-combustion chamber (Sr), wherein the gas supply channel for the pre-combustion chamber (Sr) is disposed outside the partition wall forming the pre-combustion chamber (Sr), and is connected to the upper section of the side wall forming the pre-combustion chamber (Sr), wherein the gas supply channel for the pre-combustion chamber (Sr) extends downward toward the pre-combustion chamber (Sr) from above the pre-combustion chamber (Sr), and wherein a pocket portion (50, 60) is disposed on a joint between the gas supply channel for the pre-combustion chamber (Sr) and the partition wall forming the pre-combustion chamber (Sr), the pocket portion (50, 60) including an interior space into which the ignition fuel gas flows from the gas supply channel for the pre-combustion chamber (Sr), and a communication hole (52, 62) through which the interior space and the pre-combustion chamber (Sr) communicate with each other, the communication hole (52, 62) having an opening facing in a tangent direction of the side-wall inner peripheral surface of the pre-combustion chamber (Sr); characterized in that the pocket portion (50, 60) is formed inside the partition wall forming the pre-combustion chamber (Sr). a pre-combustion chamber (Sr) disposed on a cylinder head portion; a spark plug disposed on an upper part of the pre-combustion chamber (Sr); a pre-combustion-chamber gas supply mechanism configured to supply ignition fuel gas to the pre-combustion chamber (Sr) via a gas supply channel for the pre-combustion chamber (Sr), the gas supply channel having an opening on an upper part of the pre-combustion chamber (Sr); and an opening-closing valve (24, 42) configured to open and close the gas supply channel for the pre-combustion chamber (Sr), wherein the opening of the gas supply channel for the pre-combustion chamber (Sr) is disposed on an upper wall of a partition wall forming the pre-combustion chamber (Sr) or on an upper section of a side wall of the partition wall, the opening facing in a tangent direction of a side-wall inner peripheral surface forming the pre-combustion chamber (Sr), wherein the ignition fuel gas supplied to the pre-combustion chamber (Sr) forms a first swirl flow which swirls about a longitudinal axis of the pre-combustion chamber (Sr) inside the pre-combustion chamber (Sr), wherein the gas supply channel for the pre-combustion chamber (Sr) is disposed outside the partition wall forming the pre-combustion chamber (Sr), and is connected to the upper section of the side wall forming the pre-combustion chamber (Sr), wherein the gas supply channel for the pre-combustion chamber (Sr) extends downward toward the pre-combustion chamber (Sr) from above the pre-combustion chamber (Sr), and wherein a pocket portion (50, 60) is disposed on a joint between the gas supply channel for the pre-combustion chamber (Sr) and the partition wall forming the pre-combustion chamber (Sr), the pocket portion (50, 60) including an interior space into which the ignition fuel gas flows from the gas supply channel for the pre-combustion chamber (Sr), and a communication hole (52, 62) through which the interior space and the pre-combustion chamber (Sr) communicate with each other, the communication hole (52, 62) having an opening facing in a tangent direction of the side-wall inner peripheral surface of the pre-combustion chamber (Sr); characterized in that the pocket portion (50, 60) is formed inside the partition wall forming the pre-combustion chamber (Sr). an interior space into which the ignition fuel gas flows from the gas supply channel for the pre-combustion chamber (Sr), and a communication hole (52, 62) through which the interior space and the pre-combustion chamber (Sr) communicate with each other, the communication hole (52, 62) having an opening facing in a tangent direction of the side-wall inner peripheral surface of the pre-combustion chamber (Sr); characterized in that the pocket portion (50, 60) is formed inside the partition wall forming the pre-combustion chamber (Sr).