PURPOSE: To facilitate a color measurement of a molten resin by a method wherein a color-difference meter is connected to a white corrector to be corrected to its reference.

CONSTITUTION: A light-projecting top end 22A and a lightreceiving top end 23A of a color-difference meter 21 are respectively set into a corrector light- projecting port 33Aa and a corrector light-receiving port 33Ab of a corrector 31. The color-difference meter 21 is set to a correction mode to be corrected to its reference. Next, the light-projecting top end 22A of a light-projecting optical fiber cable 22 and the light-receiving top end 23A of a light-receiving optical fiber cable 23 of the color-difference meter 21 are respectively connected to a light-projecting port and a light-receiving port of a holder 7 provided on a die 6 of a cylinder 3 of an extruder 1. The top ends come into contact with a measuring surface through a transparent body to measure a color of a flowing molten resin. In this manner, the color-difference meter 21 can measure a color constantly at high accuracy without an accumulated color measurement error only by being connected to the white corrector 31 at every measurement of a molten resin of a different color.