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By means of Raman-scattering measurement under pressures at low temperatures we study the "devil's staircase"-type phase of NaV 2 O 5 . The spin-gap mode shows a drastic softening with increasing pressure up to 0.9 GPa in C 1/4 phase and disappears between 0.9 and 1 GPa. It appears again between 1 and 2.3 GPa in C 0 phase, indicating that this phase is also a spin-gap state. Taking the charge ordering into consideration, we discuss the spin-gap states and clarify that the spin-gap mode is created by the exchange-interaction Ramanscattering mechanism and it comes from a gap between the spin-singlet ground state and the S x = 0 spin-triplet excited one. This model explains that the spin gap is almost independent of the applied magnetic field and it vanishes at about 10°lower than the critical temperature in C 1/4 phase.

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