As an enhancer for boron neutron capture therapy, a compound is anticipated which has ideal effects and physical properties in terms of the therapeutic principles of boron neutron capture therapy. As a solution, the present invention provides: a PEGylated boron cluster compound characterized in that a boron cluster having a thiol group binds to a multi-arm polyethylene glycol with a maleimide functional group; an antitumor agent including a PEGylated boron cluster compound; and an enhancer including a PEGylated boron cluster compound used for boron neutron capture therapy.