We have investigated the magnetic structure of the first field-induced phase (FM- I111) of DyCu under a magnetic field applied in the [1 1 1] direction at 2.8 K, which is below TN =63.0 K, by means of neutron diffraction experiments using a single crystal. With the magnetic transition from the antiferromagnetic (AFM) phase to the FM- I111 phase at 8.0 T, the intensity of 1 2 1- 2 0 reflection decreases sharply. The intensity in the FM- I111 phase is about three-fourths of the intensity in the AFM phase. This experimental result suggests that the magnetic structure model proposed by Amara [J. Phys. Condens. Matter 9, 7441 (1997)] is appropriate for the FM- I111 phase. Considering the present results and the previous results reported by Amara, we have quantitatively evaluated the exchange and quadrupolar interaction energies based on a mean-field approximation. These interaction energies in the AFM and FM- I111 phases are found to be comparable.