To clarify the reason for the coexistence of long-range antiferromagnetic order and metallic conductivity for NaV2O4, in which the V ions form quasi-one-dimensional zigzag chains along the b axis, we have performed a neutron-scattering experiment using a powder sample down to 20 K. The analysis of the magnetic powder diffractogram below T-N=140 K demonstrates the formation of an incommensurate spin-density-wave order with k=(0,0.191,0); the ordered moment was estimated as (0,0,0.77 mu(B)) at 20 K.