[en] A pulse-operated polarized H^- ion source of Lamb-shift type has been developed at the National Laboratory for High Energy Physics (KEK) for future acceleration of polarized protons in the KEK 12 GeV proton synchrotron. The new principle proposed by B.L. Donnally has been adopted, in which transitions of hyperfine-structure substates of 2Ssub(1/2) state to those of 2Psub(1/2) state are induced by an RF electric field in a weak magnetic field. A theoretical investigation for this principle is carried out to find the optimum condition for transitions and shows that the RF electric field should make an angle of about 50⁰ with the direction of the static magnetic field. Details of various elements and performance of the polarized ion source are also described. (auth.)