To avoid knocking without droppings engine performance by outputting gas control signal to a flow control means to control a supply of methane to the increasing side when knocking accurs on the basis of a knocking detraction signal from a knocking detecting means.

A controller 4 detects a signal out of a knocking sensor 3, judging whether there is any knocking or not. It knocking is generated, a control signal is outputted to a valve opening operation part 5a, and thereby valve opening of a flow control valve 5 is controlled to the side to increase a supply of methane. Next, on the basis of a signal out of the knocking sensor 3, whether the knocking remains or not is judged. When the knocking is continued, the valve opening is further controlled in an increasing direction, and wen the knocking vanishes, the control signal is outputted to the valve opening operation part 5a, and thereby the valve opening of the flow control valve 5 is restored to its normal state that supplies such a minimal quantity of methane as generating no knocking.