PURPOSE: To perform desired color reproduction by fetching light quantity value data from a color input machine, and performing gradation conversion on the light quantity value data by a specific gradation conversion equation.

CONSTITUTION: A color output machine (ii) which outputs a color image performs the desired color reproduction by fetching the light quantity value data from the color input machine (i) and performing the gradation conversion to the light quantity value data by a gradation conversion equation y=yH +(α(1-10-k.x)/(α-β)).(yS-yH). Where, a basic light quantity value is assumed as (x) expreseed in x=(xn-xH), the gradation intensity value of a picture element on a copy image corresponding to an arbitrary picture element on an original as (y), the gradation intensity value set in advance for H on the copy imaged corresponding to H on the original as y, the gradation intensity value set in advance for S corresponding to S on the original as y, the surface reflectivity of the recording medium of the copy image as (α), a numeric value decided by β=10-r as (β), and a numeric value decided by k=γ/(XS-XH) as (k), respectively.