A negative ion source, which utilizes a conversion of primary high current proton beam into negative ions in a gas target via charge-exchange collisions, is under development in Budker Institute, Novosibirsk. The proposed beam will be used for injection into a tandem accelerator, which is a part of the neutron source dedicated for boron-neutron capture therapy (BNCT). The ion source is designed to produce a beam that contains ≥50% of molecular ions. The initial ion beam current is about 1 A at 30 keV energy. After molecular ion dissociation in a gas target, which produces protons with an energy of 15 keV, and further charge-exchange collisions, the beam after the target will contain about 2% of negative ion species with a current in excess of 10?mA. The negative ion beam is then separated by the magnetic field, accelerated up to an energy of 105 keV and enters the tandem accelerator. This paper presents the results of simulations of the beam formation, acceleration and transport. The arrangement of the ion source and corresponding high voltage power supply are also discussed.