An in vitro method of producing an antibody having the ability of binding to CLD18A2 but not to CLD18A1 and upon binding to CLD18A2 mediating killing of cells by T cells, the method comprising the steps of: (a) transforming human host cells with an expression vector expressing the antibody heavy chain and an expression vector expressing the antibody light chain or with an expression vector expressing both chains; and (b) isolating and purifying the antibody expressed by the human host cell; wherein the transformed human host cell comprises: (i) a nucleic acid sequence encoding a polypeptide comprising the antibody heavy chain variable region comprising CDR1, CDR2, and CDR3 regions having the amino acid sequences of positions 45-52, positions 70-77, and positions 116-124 of SEQ ID NO: 117, respectively; and a nucleic acid sequence encoding a polypeptide comprising the antibody light chain variable region comprising CDR1, CDR2, and CDR3 regions having the amino acid sequences of positions 47-52, positions 70-72, and positions 109-117 of SEQ ID NO: 123, respectively; or (ii) a nucleic acid sequence encoding a polypeptide comprising the antibody heavy chain variable region comprising CDR1, CDR2, and CDR3 regions having the amino acid sequences of positions 45-52, positions 70-77, and positions 116-126 of SEQ ID NO: 116, respectively; and a nucleic acid sequence encoding a polypeptide comprising the antibody light chain variable region comprising CDR1, CDR2, and CDR3 regions having the amino acid sequences of positions 47-58, positions 76-78, and positions 115-123 of SEQ ID NO: 121, respectively; or (iii) a nucleic acid sequence encoding a polypeptide comprising the antibody heavy chain variable region comprising CDR1, CDR2, and CDR3 regions having the amino acid sequences of positions 45-52, positions 70-77, and positions 116-126 of SEQ ID NO: 118, respectively; and a nucleic acid sequence encoding a polypeptide comprising the antibody light chain variable region comprising CDR1, CDR2, and CDR3 regions having the amino acid sequences of positions 47-58, positions 76-78, and positions 115-123 of SEQ ID NO: 125, respectively. (a) transforming human host cells with an expression vector expressing the antibody heavy chain and an expression vector expressing the antibody light chain or with an expression vector expressing both chains; and (b) isolating and purifying the antibody expressed by the human host cell; wherein the transformed human host cell comprises: (i) a nucleic acid sequence encoding a polypeptide comprising the antibody heavy chain variable region comprising CDR1, CDR2, and CDR3 regions having the amino acid sequences of positions 45-52, positions 70-77, and positions 116-124 of SEQ ID NO: 117, respectively; and a nucleic acid sequence encoding a polypeptide comprising the antibody light chain variable region comprising CDR1, CDR2, and CDR3 regions having the amino acid sequences of positions 47-52, positions 70-72, and positions 109-117 of SEQ ID NO: 123, respectively; or (ii) a nucleic acid sequence encoding a polypeptide comprising the antibody heavy chain variable region comprising CDR1, CDR2, and CDR3 regions having the amino acid sequences of positions 45-52, positions 70-77, and positions 116-126 of SEQ ID NO: 116, respectively; and a nucleic acid sequence encoding a polypeptide comprising the antibody light chain variable region comprising CDR1, CDR2, and CDR3 regions having the amino acid sequences of positions 47-58, positions 76-78, and positions 115-123 of SEQ ID NO: 121, respectively; or (iii) a nucleic acid sequence encoding a polypeptide comprising the antibody heavy chain variable region comprising CDR1, CDR2, and CDR3 regions having the amino acid sequences of positions 45-52, positions 70-77, and positions 116-126 of SEQ ID NO: 118, respectively; and a nucleic acid sequence encoding a polypeptide comprising the antibody light chain variable region comprising CDR1, CDR2, and CDR3 regions having the amino acid sequences of positions 47-58, positions 76-78, and positions 115-123 of SEQ ID NO: 125, respectively. (i) a nucleic acid sequence encoding a polypeptide comprising the antibody heavy chain variable region comprising CDR1, CDR2, and CDR3 regions having the amino acid sequences of positions 45-52, positions 70-77, and positions 116-124 of SEQ ID NO: 117, respectively; and a nucleic acid sequence encoding a polypeptide comprising the antibody light chain variable region comprising CDR1, CDR2, and CDR3 regions having the amino acid sequences of positions 47-52, positions 70-72, and positions 109-117 of SEQ ID NO: 123, respectively; or (ii) a nucleic acid sequence encoding a polypeptide comprising the antibody heavy chain variable region comprising CDR1, CDR2, and CDR3 regions having the amino acid sequences of positions 45-52, positions 70-77, and positions 116-126 of SEQ ID NO: 116, respectively; and a nucleic acid sequence encoding a polypeptide comprising the antibody light chain variable region comprising CDR1, CDR2, and CDR3 regions having the amino acid sequences of positions 47-58, positions 76-78, and positions 115-123 of SEQ ID NO: 121, respectively; or (iii) a nucleic acid sequence encoding a polypeptide comprising the antibody heavy chain variable region comprising CDR1, CDR2, and CDR3 regions having the amino acid sequences of positions 45-52, positions 70-77, and positions 116-126 of SEQ ID NO: 118, respectively; and a nucleic acid sequence encoding a polypeptide comprising the antibody light chain variable region comprising CDR1, CDR2, and CDR3 regions having the amino acid sequences of positions 47-58, positions 76-78, and positions 115-123 of SEQ ID NO: 125, respectively.