In this proposal, we would like to suggest neutron diffraction experiment on an archetypical triangular lattice antiferromagnet CuFeO2 single crystal in fields up to 23T. This system shows "metamagnetic staircase” in which a series of magnetic phase transitions and a lattice-distortion-relieving transitions simultaneously take place with increasing applied magnetic field B along the c axis. The main objective is to determine the magnetic structure in the 3rd-field-induced magnetic state with 1/3 magnetization (20T < B < 34T), which is inferred to be 3SL phase, in other words, to investigate if the in-plane propagation wave vector is actually (1/3 1/3 0) and how the 3SL layers stack along the c axis. In addition, magnetic phase boundary between 3SL and intermediate-temperature PDIC phases can be obtained.