The present invention provides: an antibody or a antibody fragment thereof, which can bind to A33, which specifically attacks A33-expressing tumor cells with the use of ADCC and CDC based on the immune system, and for which no HAHA is produced; and a preventive or therapeutic agent for various malignant tumors including solid tumors that are currently treated with difficulty, which contains the antibody or an antibody fragment thereof. Specifically, the antibody or a functional fragment thereof is capable of binding to A33 and is produced by a hybridoma M10 (accession No. FERM BP-10107), M96 (accession No. FERM BP-10108), M165 (accession No. FERM BP-10106), N26 (accession No. FERM BP-10109), Q47 (accession No. FERM BP-10104), Q54 (accession No. FERM BP-10105), or R5 (accession No. FERM BP-10107). The preventive or therapeutic agent for tumors contains the antibody or a functional fragment thereof.