To provide a bi-fuel engine for a vehicle of a structure can improving the engine assembly property while sufficiently ensuring the air cleaner volume and can protecting a gaseous fuel injection device when an external force is applied on the vehicle.

The bi-fuel engine includes a plurality of intake branch pipes 51 connected with an engine body 4, a surge tank 52, an air cleaner 7, a liquid fuel injection device 8, a gaseous fuel hose 9, and a gaseous fuel injection device 11. The surge tank 52 is disposed above the engine body 4 with a space S1 enabling detachment work of a cylinder head cover 14 provided therebetween. The intake branch pipes 51 comprise a curved part 51C disposed sideways of the engine body 4 to be curved. A part from an upstream end part 51A to the curved part 51C provides an inclined part 51D inclined slantly downward from the upstream end part 51A toward the curved part 51C. The air cleaner 7 is disposed above the inclined part 51D so that the gaseous fuel injection device 11 is disposed between the air cleaner 7 and the surge tank 52.