<p>A multi-cusp DC arc-discharge hydrogen negative ion (H⁻) source has been developed for proton cyclotron, which is used for Boron Neutron Capture Therapy (BNCT). The purpose of this study is to understand the dependence of H⁻ production on the operation parameters and optimize H⁻ production in the source. In this paper, we focus on the effect of magnetic filter field on H⁻ volume production. The H⁻ density in the extraction region has been numerically estimated in the three patterns of magnetic filter field by using 3D electron transport code and 0D rate equations. The result suggests that the magnetic filter field which is localized in the vicinity of the extraction region is suitable for the efficient H⁻ volume production.</p>