To guide an engine to a fastening position without restricting the flexibility of design.

In the mount structure, the engine is inserted between a pair of side members 12, and a horizontal supporting plate 14 is provided on the engine side of the side members. A mount 21 opposing to the supporting plate is attached to a side portion of the engine, and an auxiliary plate 18 formed with a reversed conical guide hole 18a is laminated and bonded on an upper surface of the supporting plate. The mount is lowered to insert a projecting portion 23b projecting from the mount into the guide hole. Upward standing pieces 14a are respectively formed at both ends in a fore-and-aft direction of the supporting plate 14. The mount 21 is formed so as to be inserted between the standing pieces when a tip end of the projecting portion 23a is positioned on an upper side in a width direction of the guide hole 18a. A tilting portion 23d moving the mount so as to be inserted between the standing pieces is formed on the mount.