To provide a single cylinder gas engine which sufficiently utilizes the engine ability by carrying out a preferable actuator control for restraining response delay and overshoot of rotation speed with respect to a target rotation speed change at load variation.

The single cylinder gas engine 1 includes an engine body 2, control actuators 35, 53 for adjusting the amounts of a fuel gas and air supplied to the engine body 2, and a control device 9 for controlling the control actuators 35, 53 according to load variation. The control device 9 makes the control period TC for controlling the control actuators 35, 53 variable according to the rotation speed of the engine body 2 (average rotation speed N) and controls the control actuators 35, 53 at a frequency of 1 to 3 times per operation cycle (cycle period T) of the engine body 2.