PURPOSE: To simplify the arrangement of a fuel gas feed pipe, to prevent the discharge of unburnt gas during the stop of an engine, and to prevent the occurrence of a delay in a starting time during restarting.

CONSTITUTION: A fuel gas feed source 2 is communicated with the intake passage 7 of a fuel gas engine 1, in the order, through a pressure regulator 3, a flow rate regulator 4, a fuel gas feed pipe 5, and a mixer 6. A valve seat 9 is formed at the opening end 5a of the fuel gas feed pipe 5 opened in the mixer 6, a fuel gas cut-off valve 8 is arranged facing the opening end 5a, and the valve body 8a of the fuel gas cut-off valve 8 is brought into contact with the valve seat 9. The valve body 8a of the fuel gas cut-off valve 8 is operable by means of an actuator 10 arranged to the opposite wall 6a of the opening end 5a of the mixer 6.