An antibody which binds to CLD18A2 and which does not bind to CLD18A1, wherein the antibody, upon binding to CLD18A2 mediates killing of cells expressing CLD18A2 by ADCC or CDC, wherein the antibody comprises: (i) an antibody heavy chain 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 an antibody light chain 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 said antibody chains with up to 5 amino acid substitutions in said CDR region; or (ii) an antibody heavy chain 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 an antibody light chain 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 said antibody chains with up to 5 amino acid substitutions in said CDR region; or (iii) an antibody heavy chain 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 an antibody light chain 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; or said antibody chains with up to 5 amino acid substitutions in said CDR region. (i) an antibody heavy chain 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 an antibody light chain 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 said antibody chains with up to 5 amino acid substitutions in said CDR region; or (ii) an antibody heavy chain 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 an antibody light chain 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 said antibody chains with up to 5 amino acid substitutions in said CDR region; or (iii) an antibody heavy chain 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 an antibody light chain 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; or said antibody chains with up to 5 amino acid substitutions in said CDR region.