[en] The whole region of superconductivity in the Ba_1-xK_xBiO₃ (BKBO) system (0.35< x<1) has been investigated by synthesizing the samples with the high pressure and high temperature technique. As the K doping ratio (x) increases, the superconducting transition temperature (T_c) of the BKBO system decreases from 30.4 K for x=0.37 to very low diamagnetic signal below 3 K for x=0.74. The synthesis of the samples without including any bariums in the starting composition (x=1) resulted in different T_c and lattice parameters from sample to sample with partial substitutions of Bi at the K site in final products. With detailed synthesis information of the BKBO samples which varies with x, the evolution of T_c by doping in the cubic BKBO system is compared with other superconducting bismuthates and discussed on a basis of its electronic band structure