1. A method of measuring a radial gradient index distribution n(r) of a rod lens by calculating higher-order index distribution coefficients indicating said gradient index distribution n(r) when n(r) is given by the expression: 2 n &af; ( r ) 2 = &it; n 0 2 &CenterDot; { 1 - ( g &CenterDot; r ) 2 + h 4 &af; ( g &CenterDot; r ) 4 + h 6 &af; ( g &CenterDot; r ) 6 + h 8 &af; ( g &CenterDot; r ) 8 + &ctdot; } in which r is a radial distance measured from an optical axis, n 0 is a refractive index on the optical axis, g is a secondary index distribution coefficient, and h 4, h 6 and h 8 are higher-order index distribution coefficients, said method comprising the steps of: (1) processing said rod lens so that an optic-axial length of said rod lens is approximately equal to P/2 (in which P represents a paraxial period length (pitch) defined as P&equals;2&pgr;/g) or approximately equal to an integer multiple of P/2 and so that opposite end surfaces of said rod lens are shaped like parallel planes; (2) setting a patterned surface as an object surface in the proximity of one end surface of said rod lens and forming an image surface in the proximity of the other end surface of said rod lens by irradiating said patterned surface with condensed monochromatic light; (3) obtaining the position of a paraxial focal point and the curve of curvature of field by observing said image surface; and (4) calculating back higher-order index distribution coefficients by a fitting process on the basis of said position of the paraxial focal point and said curve of curvature of field.