The present invention provides: antibodies specifically reacting against hDlk-1 and having anti-tumor activity in vivo (anti-hDlk-1 antibodies, and in particular, humanized anti-hDlk-1 antibodies); fragments of the antibodies; hybridomas that produce the antibodies; a complex of the antibody or antibody fragment and an agent; a pharmaceutical composition, a tumor therapeutic agent, a tumor diagnostic agent and an agent for inducing apoptosis in tumor cells, each of which comprises the aforementioned antibody or the like; a method for treating tumor, a method for detecting tumor, a method for inducing apoptosis in tumor cells, a kit for detecting and/or diagnosing tumor and a kit for inducing apoptosis in tumor cells, each of which comprises the use of the aforementioned antibody or the like; etc.