The present invention relates to HER2 (Human Epidermal Growth Factor Receptor 2) antibodies to prevent or treat cancers. The antibodies of the invention binds specifically to HER2 over-expressed in cancer cells (particularly, breast cancer and stomach cancer cells), specifically to an epitope on HER2 being different from epitope for trastuzumab. The CDR sequences of the present antibodies exhibit low similarity to CDR sequences of publicly known HER2 antibodies, addressing that the CDR sequences are unique. The antibodies of the present invention in combination with trastuzumab kill cancer cells with significantly enhanced cytotoxicity and therefore very effective in therapy of cancer (particularly, breast cancer and stomach cancer). Without wishing to be bound by theory, the enhanced efficacies of the combined therapy would address that the antibodies of the present invention bind to epitope on HER2 being different from epitope for trastuzumab, and inhibit HER2 in a cooperative manner with trastuzumab.