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 activityofsuppressing phosphorylation of an ErbB3 protein in a cancer cell, and an activity of suppressing in vivo tumor proliferation.