Device and method for adjusting fuel for gas engines

The purpose of the present invention is to efficiently operate gas engines by ascertaining the properties of a fuel gas in advance and eliminating the need to change gas engine specifications even if the properties of the fuel gas from a fuel supply source change. A device for adjusting fuel for gas engines comprises: a fuel gas line that sends fuel gas that is from a fuel supply source to a gas engine fuel supply valve; a non-combustible gas line, which is connected to the fuel gas line, and whereby the fuel gas in the fuel gas line is mixed with non-combustible gas; a flow rate adjusting valve that adjusts the flow rate of the non-combustible gas flowing from the non-combustible gas line into the fuel gas line; a properties detector for detecting the properties of the fuel gas flowing further downstream than the junction point in the fuel gas line where the non-combustible gas line is connected; and a controller that controls the degree the flow rate adjusting valve is opened so that the value detected by the properties detector approaches a prescribed target value.