Of all multiferroic materials, BiFeO3 stands out particularly with its both magnetic and ferroelectric transition temperatures above room temperatures, and more interestingly for its incommensurate magnetic structure with an extremely long period of 630 A. Despite its attractive magnetic properties, information is almost non-existent of the microscopic spin Hamiltonian. With using co-aligned single crystals of BiFeO3, we have succeeded in measuring the spin wave of BiFeO3 by employing MAPS first and AMATERAS of J-PARC more recently. Although it was successful in its own right, we have not yet obtained the full spin wave from the two experiments because of the three-dimensional nature of the spin wave. In order to complete and obtain the 3D full spin wave, here we propose to measure BiFeO3 by rotating the sample at MERLIN as have been demonstrated for several three-dimensional systems.