PURPOSE:To obtain a high-resolution encoder by forming a code pattern provided to a disk by using a code pattern which varies continuously in width on a constant-radius circle symmetrically between the outside direction and inside direction. CONSTITUTION:The code pattern provided to the disk 1 is formed of the code pattern which varies in width continuously on the constant-radius circle between the outside direction and inside direction. Then, light beams transmitted through or reflected by the code pattern are detected by a light receiving part 5 and the quantity of light reception of the light receiving part 5 is proportional to the light transmission or reflection area of the code pattern is a light receiving window 4a; when the disk 1 rotates and the light transmission or reflection area of the code pattern in the light receiving window 4a varies continuously, the quantity of light reception of the light receiving part 5 is proportional to the variation of the code pattern, and consequently the level of the signal outputted by the light receiving part 5 varies, so that the rotating speed or position, etc., of the disk is detected with high accuracy.
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