Loop fusion is often necessary before successful application of high-level synthesis (HLS). Although promising loop optimization tools based on the polyhedral model such as Pluto have been proposed, they sometimes cannot fuse loops into fully nested loops. This paper proposes an effective loop transformation called Outer Loop Shifting (OLS) that facilitates successful loop fusion. With HLS, we found that the OLS generates hardware with 25% less execution cycles on average than that only by Pluto for four benchmark programs.