A hole saw includes a shaft, a hole-cutter and an adjustable outer cutter unit. The shaft has a bottom coupling hole for receiving a pilot drill, a top coupling rod for fastening to a driving tool, and an outer thread extending around the periphery thereof. The hole-cutter has a top center inner thread threaded onto the outer thread of the shaft. The adjustable outer cutter unit has two locating rods perpendicularly affixed to the shaft at two sides in a line and two cutters respectively movably mounted on the locating rods, locked thereto with a respective lock screw and abutted against an outer periphery of said hole-cutter.