  We present the results of 0.6"-resolution observations of CO J=3-2 line
emission in 10 massive star-forming galaxies at z=2.2-2.5 with the Atacama
Large Millimeter/submillimeter Array (ALMA). We compare the spatial extent of
molecular gas with those of dust and stars, traced by the 870 $\mu$m and 4.4
$\mu$m continuum emissions, respectively. The average effective radius of the
CO emission is 1.75$\pm$0.34 kpc, which is about 60 percent larger than that of
the 870 $\mu$m emission and is comparable with that of the 4.4 $\mu$m emission.
Utilizing the best-fit parametric models, we derive the radial gradients of the
specific star-formation rate (sSFR), gas depletion timescale, and gas-mass
fraction within the observed galaxies. We find a more intense star-formation
activity with a higher sSFR and a shorter depletion timescale in the inner
region than in the outer region. The central starburst may be the primary
process for massive galaxies to build up a core. Furthermore, the gas-mass
fraction is high, independent of the galactocentric radius in the observed
galaxies, suggesting that the galaxies have not begun to quench star formation.
Given the shorter gas depletion timescale in the center compared to the outer
region, quenching is expected to occur in the center first and then propagate
outward. We may be witnessing the observed galaxies in the formation phase of a
core prior to the forthcoming phase of star formation propagating outward.
