[en] Recently Nakai et al. reported a theoretical H-T phase diagram of flux line lattice (FLL) structure in which successive transitions from a triangular, a square (□_v), a triangular and another square (□_g) occur with increasing a magnetic field. Here □_v and □_g indicate the FLL structures reflecting anisotropies in the Fermi velocity and the superconducting gap, respectively. In the case of YNi₂B₂C, □_v and □_g should rotate by 45deg. The low field transition from triangular to □_v is observed in RENi₂B₂C (RE=Er, Tm, Lu, and Y). However, there is no experimental evidence for the appearance of □_g phase so far. We studied the FLL structure of YNi₂B₂C in the higher field region by small-angle neutron scattering. Our results show that a large area of the H-T phase diagram is occupied by □_v phase and there is no evidence for the appearance of □_g lattice. (author)