The present invention provides an agent for the prophylaxis or therapy of autoimmune diseases or allergic diseases, which contains an anti-Embigin antibody, particularly an anti-Embigin antibody showing cytotoxicity or a cytotoxicity inducing activity, an agent for the prophylaxis or therapy of diseases involving Th17 cell, and a cytotoxic agent to Th17 cell. In addition, an agent for detection of Th17 cell, which contains an anti-Embigin antibody, a convenient detection method of Th17 cell, which uses the agent, a method of efficiently delivering a drug and the like in a Th17 cell selective manner, which uses an anti-Embigin antibody, and a drug delivery system to Th17 cell are provided.