Abstract: An eccentricity measurement method of measuring the eccentricity between a center of a connection portion of a rotary disk of a rotary encoder connected to a rotary shaft of a rotary member, and a center of a scale provided at the periphery of the rotary disk. The method includes forming, on the rotary disk, a circle pattern concentric with the scale and at least three straight-line patterns intersecting or contacting the circle pattern and the connection portion; and measuring an amount and direction of the eccentricity of the rotary disk by comparing a coordinate of the center of the connection portion, calculated from coordinates of intersections of the straight-line patterns and the connection portion, with a coordinate of the center of the scale, calculated from the coordinates of intersections of the straight-line patterns and the circle pattern.