A magnetic encoder constituting rotation detector in combination with a magnetic sensor, comprising a circular multipolar magnet and a metal reinforcing ring to which the circular multipolar magnet is fixed is disclosed.The encoder is characterized by a projecting portion which is provided at a surface of the circular multipolar magnet facing to the metal reinforcing ring, so as to form an adhesive space to secure uniform adhesive layer between metal reinforcing ring and the circular multipolar magnet, thereby preventing from being peeled off or cracked. And the height of the projecting portion is adapted to define the thickness of the adhesive layer.