  We report bulk superconductivity induced by an isovalent doping of phosphorus
in BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$. The P-for-As substitution results in
shrinkage of lattice, especially for the FeAs block layers. The resistivity
anomaly associated with the spin-density-wave (SDW) transition in the undoped
compound is gradually suppressed by the P doping. Superconductivity with the
maximum $T_c$ of 30 K emerges at $x$=0.32, coinciding with a magnetic quantum
critical point (QCP) which is evidenced by the disappearance of SDW order and
the linear temperature-dependent resistivity in the normal state. The $T_c$
values were found to decrease with further P doping, and no superconductivity
was observed down to 2 K for $x\geq$ 0.77. The appearance of superconductivity
in the vicinity of QCP hints to the superconductivity mechanism in iron-based
arsenides.
