  Color gradients in elliptical galaxies in distant clusters ($z=0.37-0.56$)
are examined by using the archival deep imaging data of Wide Field Planetary
Camera 2 (WFPC2) on-board the Hubble Space Telescope (HST). Obtained color
gradients are compared with the two model gradients to examine the origin of
the color gradients. In one model, a color gradient is assumed to be caused by
a metallicity gradient of stellar populations, while in the other one, it is
caused by an age gradient. Both of these model color gradients reproduce the
average color gradient seen in nearby ellipticals, but predict significantly
different gradients at a redshift larger than $\sim$0.3. Comparison between the
observed gradients and the model gradients reveals that the metallicity
gradient is much more favorable as the primary origin of color gradients in
elliptical galaxies in clusters. The same conclusion has been obtained for
field ellipticals by using those at the redshift from 0.1 to 1.0 in the Hubble
Deep Field-North by Tamura et al. (2000). Thus, it is also suggested that the
primary origin of the color gradients in elliptical galaxies does not depend on
galaxy environment.
