To increase a compression ratio by using gaseous fuel, and to enhance the thermal efficiency by enhancing the availability rate of air in a main chamber.

This gas engine is composed of a counter chamber member 3 and a combustion chamber member 10 arranged in a cylinder head 7, a gas introducing valve 5 arranged in a gas introduction port 23 for supplying the gaseous fuel into a counter chamber 2, and a control valve 4 for opening and closing communication holes 13 for communicating a main chamber 1 with the counter chamber 2. A part of the communication holes 13 is opened to valve recesses 30, 31 formed in the combustion chamber member 10, the remaining part of the communication holes 13 is communicated with a guide groove 21. Protrusion 20 fitted in the guide groove 21 in the combustion chamber member 10 are formed in the top surface 39 of the piston. Communication passages 41 radially extended around the periphery of the cylinder surrounded by the guide groove 21 and the protrusions 20 are formed in the vicinity of the top dead center of the piston.