We study the curvaton scenario using the MSSM flat directions in the
gauge-mediated SUSY breaking model. We find that the fluctuations in the both
radial and phase directions can be responsible for the density perturbations in
the universe through the curvaton mechanism. Although it has been considered
difficult to have a successful curvaton scenario with the use of those flat
directions, it is overcome by taking account of the finite temperature effects,
which induce a negative thermal logarithmic term in the effective potential of
the flat direction.Comment: 12 page