BiFeO3, a rare multiferroic compound that shows antiferromagnetism and ferroelectricity simultaneously above room temperature (TN~640 K), has been studied intensively. Recently, a new magnetic phase has been found between the cycloidal and canted antiferromagnetic phase in magnetic field. This intermediate phase is considered to originate from the anisotropic magnetic interactions that are coupled to the ferroelectric property. It is important to clarify the magnetic structure to fully understand the multiferroic behavior in this material. We propose to measure the magnetic field dependence of some magnetic Bragg peaks up to 18 T.