To provide a fuel economy improvement device which is improved in assembling workability and maintenance workability and which can exhibits desired performance.

This fuel economy improvement device improves the fuel economy of a combustion device by oppositely arranging magnets in a hollow supporting frame mounted in the intake pipe of the combustion device and applying magnetic force of the magnets to oxygen led to flow into the combustion device from the intake pipe. The magnets are detachably mounted to the supporting frame through a holder retaining the magnets. The holder is so configured that it is bent inside out across the supporting frame and a pair of magnets is retained with different polarities opposed to each other on both surfaces. Also, the holder is so configured that a bent portion bent inside out is positioned on the upstream side of the supporting frame, and inserted and mounted to the supported frame from upstream of the supporting frame toward downstream thereof.