The magnetic field effect on the phase diagram of the organic Mott system κ-(BEDT-TTF)2Cu[N(CN)2]Br in which the bandwidth was tuned by the substitution of deuterated molecules was studied by means of the resistivity measurements performed in magnetic fields. The lower critical point of the first-order Mott transition, which ended on the upper critical field Hc2-temperature plane of the superconductivity, was determined experimentally in addition to the previously observed upper critical end point. The lower critical end point moved to a lower temperature with the suppression of Tc in magnetic fields and the Mott transition recognized so far as the S-shaped curve reached T = 0 when H > Hc2 in the end.PACS numbers: 74.70. Kn, 71.30.+h, 74.25.Dw Metal-insulator (MI) transitions have been one of the central issues in strongly correlated electron systems. Among the various types of MI transitions, the Mott transition is the most attractive phenomenon, which arises from an electron-electron interaction in a wide range of materials.1 Mott insulator derives from a large electrical Coulomb repulsion energy U with respect to the bandwidth W . Metal transitions can occur in two ways in a Mott insulator: one can change the strength of the interaction U/W , while maintaining the required bandfilling value and is referred to as a bandwidth-controlled Mott transition or one can introduce carriers with the required density, while maintaining U/W and is referred to as a band-filling-controlled Mott transition. The former case occurs, for example, in vanadium oxides and molecular conductors, while the latter occurs in high-T c cuprate superconductors.Organic charge transfer salts based on the donor molecule bis(ethylenedithio)-tetrathiafulvalene, abbreviated as BEDT-TTF or ET, have been recognized as a highly correlated electron system.2 Among them, κ-(ET) 2 X with X = Cu(NCS) 2 , Cu[N(CN) 2 ]Y (Y = Br and Cl), etc. has attracted considerable attention due to its strongly correlated quasi-two-dimensional electron system because the strong dimer structure consisting of two ET molecules makes the conduction band half filled.3,4 Furthermore, the softness of the lattice in molecular conductors enables us to easily modulate the bandwidth by applying small physical and chemical pressures, while maintaining the band-filling. Therefore, the κ-(ET) 2 X system has been considered to be one of the typical bandwidth-controlled Mott systems.The first-order Mott transition characterized by an Sshaped curve induces the correlated metal and superconductor phases from the Mott insulator phase by applying a small helium gas pressure or by the slight molecular substitution of the donor or anion molecules. 3,5,6,7,8 The relation and transitions among the various phases in the phase diagram have been studied extensively, both experimentally and theoretically. Several unique features have been found in association with the Mott transition. The first-order transition is terminated at T U cr ≃ 32- 40 K 6,8,9 , which is the upper critical end poin...