A neutron absorber synthesis system that can synthesize boron carbide that is a raw material for a neutron absorber, by recycling boron (B-10) of a mass number 10 that can absorb boron, particularly neutrons existing in boric acid waste fluid, is provided. The neutron absorber synthesis system includes: a pre-processing unit to which a radioactive waste including boron is supplied from the outside and inflows to the inside and a compound is produced by removing moisture of the radioactive waste by heat treatment by a first heat source; and a boron carbide synthesizing unit to which the compound produced from the radioactive waste is inflowed inside and a boron carbide is synthesized from a raw material containing the compound and carbon by heat treatment by a second heat source.