PROBLEM TO BE SOLVED: To avoid the occurrence of run-on after key OFF, in a gas fuel supply device having a vacuum type lock mechanism. SOLUTION: This gas fuel supply device of an engine has a vaporizer 5 for decompressing and vaporizing gas fuel and a mixer 23 sending out the decompressed gas fuel to an intake pipe duct 27, and a negative pressure introducing chamber 20 connected to the downstream side of a throttle valve 23A of the intake pipe duct 27 in the vaporizer 5 by an air passage 22A, and has the vacuum type lock mechanism for cutting off a fuel passage by introducing a pressure variation in the intake pipe duct 27 by stopping of the engine 26. The upstream side of the throttle valve 23A of an intake passage 27 is connected to a midway part of a gas fuel passage 14 connecting the vaporizer 5 and the mixer 23 by an air passage 22B having a second solenoid valve 30, and upstream side air of the throttle valve 23A is introduced into the gas fuel passage 14 by opening the solenoid valve 30 in key OFF. COPYRIGHT: (C)2008,JPO&INPIT