The elastic properties of a filled skutterudite PrOs4Sb12 with a heavy fermion superconductivity at T-C=1.85 K have been investigated. The elastic softening of (C-11-C-12)/2 and C-44 with lowering temperature down to T-C indicates that the quadrupolar fluctuation due to the CEF state plays a role for the Cooper paring in superconducting phase of PrOs4Sb12. A Debye-type dispersion in the elastic constants around 30 K revealed a thermally activated Gamma(23) rattling due to the off-center Pr-atom motion obeying tau=tau(0) exp(E/k(B)T) with an attempt time tau(0)=8.8x10(-11) s and an activation energy E=168 K. It is remarkable that the charge fluctuation of the off-center motion with Gamma(23) symmetry may mix with the quadrupolar fluctuation and enhance the elastic softening of (C-11-C-12)/2 just above T-C.