An object is to provide an antibody capable of specifically recognizing a human NRG1 protein isoform, and suppressing signal transduction in which the isoform is involved. An antibody capable of binding to a region at positions 221 to 234 of a human NRG1-α protein or an antibody capable of binding to a region at positions 213 to 239 of a human NRG1-β1 protein was successfully obtained. Further, it was also found that these antibodies had an activity of suppressing cleavage of the NRG1 protein, an activity of suppressing phosphorylation of an ErbB3 protein in a cancer cell, and an activity of suppressing in vivo tumor proliferation.