A monoclonal antibody to KIR2DS1 or a fragment containing an antigen-binding region thereof has VL having an amino acid sequence of SEQ ID NO: 1 or an amino acid sequence having the same amino acid sequence as the amino acid sequence except that one or several amino acids are conservatively substituted as CDR1, having an amino acid sequence of SEQ ID NO: 2 or an amino acid sequence having the same amino acid sequence as the amino acid sequence except that one or several amino acids are conservatively substituted as CDR2, and having an amino acid sequence of SEQ ID NO: 3 or an amino acid sequence having the same amino acid sequence as the amino acid sequence except that one or several amino acids are conservatively substituted as CDR3; and VH having an amino acid sequence of SEQ ID NO: 4 or SEQ ID NO: 5 or an amino acid sequence having the same amino acid sequence as the amino acid sequence except that one or several amino acids are conservatively substituted as CDR1, having an amino acid sequence of SEQ ID NO: 6 or an amino acid sequence having the same amino acid sequence as the amino acid sequence except that one or several amino acids are conservatively substituted as CDR2, and having an amino acid sequence of any one selected from the group consisting of SEQ ID NO: 7, SEQ ID NO: 8, and SEQ ID NO: 9 or an amino acid sequence having the same amino acid sequence as the amino acid sequence except that one or several amino acids are conservatively substituted as CDR3.