US Patent 7508193 Encoder

To easily manufacture an encoder, in which N-poles and S-poles of magnets are arranged alternately in a circumferential direction and in which an unequal pitch portion is provided in a part on a circumference, without using a magnetizing head of a special shape having an unequal pitch portion, the unequal pitch portion is formed by arranging the N-poles and the S-poles at an equal pitch all over the circumference and then providing a part on the circumference with a shielding member for shielding a magnetism. Alternatively, the unequal pitch portion is formed by arranging the N-poles and the S-poles at an equal pitch all over the circumference and then providing a part on the circumference with a cut portion.