Provided is an antitumor agent targeting SIRP? which inhibits binding between CD47 and SIRP? the antitumor agent being more effective. The present invention also provides an antitumor agent capable of more effectively exhibiting an antitumor effect when used in combination with an immune checkpoint inhibitor or an antibody drug. The antitumor agent includes as an active ingredient a substance that molecularly targets an IgV domain, which is an extracellular domain of SIRP?The antitumor agent of the present invention, including as an active ingredient a substance that molecularly targets an IgV domain of SIRP?protein, activates M1-type macrophages, which have cytotoxicity to cancer cells, and immunocompetent cells to provide an effective antitumor effect. Further, the antitumor agent can effectively exhibit an antitumor action not only on cancer cells expressing SIRP?on a cell surface but also on cancer cells not expressing the SIRP?when used in combination with, for example, an immune checkpoint inhibitor and/or an antibody drug that specifically reacts with a cancer antigen and has ADCC and ADCP activities.