The off-center rattling of Nd rare-earth ions in the filled skutterudite compound NdOs4Sb12 has been investigated by ultrasonic measurement. The longitudinal C-11 mode for frequencies between 34 and 253MHz shows a marked frequency dependence in elastic constant and ultrasonic attenuations at two different temperatures centered at around 45 and 15K. The relaxational frequency dependence of ultrasonic dispersion reveals a thermally activated Gamma(23)-type off-center motion of Nd-ions, involving local charge fluctuations with Gamma(23) symmetry in the (OsSb3)(4) cage. An attempt time tau(0,(1)) = 7.5 x 10(-12) s and an activation energy E-1 = 337 K were obtained from fits to the dispersion at around 45 K. The presence of another dispersion at lower temperatures of around 15 K implies that the rattling in NdOs4Sb12 has an additional low-energy excitation characterized by a lower activation energy E-2 similar to 67 K with an attempt time tau(0,(2)) = 5.1 x 10(-11) s.