The origin of color gradients in elliptical galaxies is examined by comparing model gradients with those observed in the Hubble Deep Field. The models are constructed so as to reproduce color gradients in local elliptical galaxies either by a metallicity gradient or by an age gradient. By looking-back a sequence of the color gradient as a function of redshift, the age-metallicity degeneracy is solved. The observed color gradients in elliptical galaxies at z = 0.1 to 1.0 agree excellently with those predicted by the metallicity gradient, while they deviate significantly from those predicted by the age gradient even at z ~ 0.3, and the deviation becomes larger with increasing redshift. This result does not depend on cosmological parameters and parameters for an evolutionary model of an elliptical galaxy within a reasonable range. Thus our results clearly indicate that the origin of color gradients is not age but stellar metallicity.
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