A linearizing correction unit (104) carries out a linearizing correction process on the output of an image sensor (8) based upon linearizing correction data stored in a linearizing correction data holding unit (102), and a light-irregularity correction unit (108) carries out a light-irregularity correction process on the image sensor output that has been subjected to the linearizing correction process based upon light-irregularity correction data stored in a light-irregularity correction data holding unit (106). A refection factor calculation unit (110) calculates an integral value of the in-plane reflection factor of a test piece by using the output that has been subjected to the linearizing correction and light-irregularity correction with respect to pixel outputs of the image sensor (8) obtained when the test piece having in-plane density irregularities is measured. A quantifying unit (114) applies calibration curve data of a calibration-curve-data holding unit (112) to the integrated reflection factor obtained by the reflection factor calculation unit so that a sample density of the test piece is calculated.