A diamond sintered body conventionally used in a cutting tool or the like includes an iron group metal element as a sintering aid, and therefore has a problem in heat resistance. A diamond sintered body not including the iron group metal, on the other hand, does not have sufficient mechanical strength to be used as a tool material, and also does not have conductivity, which makes electrical discharge machining impossible, and thus processing thereof is difficult. A diamond polycrystalline body having high heat resistance and mechanical strength and having conductivity enabling electrical discharge machining is obtained by using only an amorphous or fine graphite-type carbon material as a starting material, adding boron thereto and concurrently performing conversion into diamond and sintering in an ultra-high pressure and temperature condition.