A pharmaceutical composition for cancer treatment, which comprises a monoclonal antibody or an antigen-binding fragment thereof, wherein the monoclonal antibody binds to a three-dimensionally structured epitope within the extracellular region of SLC6A6 and is at least one selected from the group consisting of: (a) an antibody which comprises a heavy chain variable region comprising amino acid sequences located at positions 50 to 54, positions 69 to 86 and positions 118 to 125 in SEQ ID NO: 24 as heavy chain CDR1, CDR2 and CDR3, respectively, and a light chain variable region comprising amino acid sequences located at positions 44 to 54, positions 70 to 76 and positions 109 to 116 in SEQ ID NO: 26 as light chain CDR1, CDR2 and CDR3, respectively; (b) an antibody which comprises a heavy chain variable region comprising amino acid sequences located at positions 50 to 54, positions 69 to 86 and positions 118 to 126 in SEQ ID NO: 28 as heavy chain CDR1, CDR2 and CDR3, respectively, and a light chain variable region comprising amino acid sequences located at positions 46 to 57, positions 73 to 79 and positions 112 to 119 in SEQ ID NO: 30 as light chain CDR1, CDR2 and CDR3, respectively; (c) an antibody which comprises a heavy chain variable region comprising amino acid sequences located at positions 50 to 54, positions 69 to 86 and positions 118 to 128 in SEQ ID NO: 32 as heavy chain CDR1, CDR2 and CDR3, respectively, and a light chain variable region comprising amino acid sequences located at positions 48 to 58, positions 74 to 80 and positions 113 to 120 in SEQ ID NO: 34 as light chain CDR1, CDR2 and CDR3, respectively; (d) an antibody which comprises a heavy chain variable region comprising amino acid sequences located at positions 50 to 54, positions 69 to 86 and positions 118 to 131 in SEQ ID NO: 36 as heavy chain CDR1, CDR2 and CDR3, respectively, and a light chain variable region comprising amino acid sequences located at positions 44 to 54, positions 70 to 76 and positions 109 to 116 in SEQ ID NO: 38 as light chain CDR1, CDR2 and CDR3, respectively; (e) an antibody which comprises a heavy chain variable region comprising amino acid sequences located at positions 50 to 54, positions 69 to 86 and positions 118 to 129 in SEQ ID NO: 40 as heavy chain CDR1, CDR2 and CDR3, respectively, and a light chain variable region comprising amino acid sequences located at positions 44 to 54, positions 70 to 76 and positions 109 to 116 in SEQ ID NO: 42 as light chain CDR1, CDR2 and CDR3, respectively; and (f) an antibody which comprises a heavy chain variable region comprising amino acid sequences located at positions 50 to 54, positions 69 to 86 and positions 118 to 126 in SEQ ID NO: 44 as heavy chain CDR1, CDR2 and CDR3, respectively, and a light chain variable region comprising amino acid sequences located at positions 46 to 57, positions 73 to 79 and positions 112 to 119 in SEQ ID NO: 46 as light chain CDR1, CDR2 and CDR3, respectively. (a) an antibody which comprises a heavy chain variable region comprising amino acid sequences located at positions 50 to 54, positions 69 to 86 and positions 118 to 125 in SEQ ID NO: 24 as heavy chain CDR1, CDR2 and CDR3, respectively, and a light chain variable region comprising amino acid sequences located at positions 44 to 54, positions 70 to 76 and positions 109 to 116 in SEQ ID NO: 26 as light chain CDR1, CDR2 and CDR3, respectively; (b) an antibody which comprises a heavy chain variable region comprising amino acid sequences located at positions 50 to 54, positions 69 to 86 and positions 118 to 126 in SEQ ID NO: 28 as heavy chain CDR1, CDR2 and CDR3, respectively, and a light chain variable region comprising amino acid sequences located at positions 46 to 57, positions 73 to 79 and positions 112 to 119 in SEQ ID NO: 30 as light chain CDR1, CDR2 and CDR3, respectively; (c) an antibody which comprises a heavy chain variable region comprising amino acid sequences located at positions 50 to 54, positions 69 to 86 and positions 118 to 128 in SEQ ID NO: 32 as heavy chain CDR1, CDR2 and CDR3, respectively, and a light chain variable region comprising amino acid sequences located at positions 48 to 58, positions 74 to 80 and positions 113 to 120 in SEQ ID NO: 34 as light chain CDR1, CDR2 and CDR3, respectively; (d) an antibody which comprises a heavy chain variable region comprising amino acid sequences located at positions 50 to 54, positions 69 to 86 and positions 118 to 131 in SEQ ID NO: 36 as heavy chain CDR1, CDR2 and CDR3, respectively, and a light chain variable region comprising amino acid sequences located at positions 44 to 54, positions 70 to 76 and positions 109 to 116 in SEQ ID NO: 38 as light chain CDR1, CDR2 and CDR3, respectively; (e) an antibody which comprises a heavy chain variable region comprising amino acid sequences located at positions 50 to 54, positions 69 to 86 and positions 118 to 129 in SEQ ID NO: 40 as heavy chain CDR1, CDR2 and CDR3, respectively, and a light chain variable region comprising amino acid sequences located at positions 44 to 54, positions 70 to 76 and positions 109 to 116 in SEQ ID NO: 42 as light chain CDR1, CDR2 and CDR3, respectively; and (f) an antibody which comprises a heavy chain variable region comprising amino acid sequences located at positions 50 to 54, positions 69 to 86 and positions 118 to 126 in SEQ ID NO: 44 as heavy chain CDR1, CDR2 and CDR3, respectively, and a light chain variable region comprising amino acid sequences located at positions 46 to 57, positions 73 to 79 and positions 112 to 119 in SEQ ID NO: 46 as light chain CDR1, CDR2 and CDR3, respectively.