Vortex phase transition with decoupling of the adjacent layers in the organic superconductor $\kappa -{\mathrm{}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F})\mathrm{}}_2{\mathrm{Cu}\left[\mathrm{N}\right(\mathrm{CN})\mathrm{}}_2]\mathrm{Br}$

The temperature dependence of the local magnetic induction measured by using a miniature Hall probe shows a tiny step at a field-dependent temperature in the titled compound. This result implies that there exists a first-order phase transition in the vortex system in this organic superconductor as reported in high-$T_c$ cuprates. The height of the step is of the order of 10 mG, from which the entropy change at the transition is estimated to be about 0.1 ($k_B$/vortex)/layer. The measurements of the Josephson plasma resonance reveal that the interlayer phase coherence abruptly changes across the transition line, indicating the decoupling of the adjacent layers.