Provided is a novel boron-containing compound. A compound represented by formula below is prepared and used. A black circle represents a B atom, and a white circle represents B-H； -R1 represents a group having 3-10 repeating sequences of -(CH2)n-X1-R3 (n represents an integer of 0-6； X1 represents O, S, NH, S-S, O-CO, NHCO, or SCO, or is absent； and R3 represents C6-C20 alkyl, hydroxy C6-C20 alkyl, amino C6-C20 alkyl, azido C6-C20 alkyl, hydroxycarbonyl C6-C20 alkyl, or the like) or -(CH2)2-O-, and having a methyl group or an ethyl group at the end of the oxygen atom side； and -R2 represents -(CH2)m-X2-R4 (m represents an integer of 0-8； X2 represents O, S, NH, S-S, O-CO, NHCO, or SCO, or is absent； and R4 represents a tumor recognition site) or is absent.
