Endogenous ligands of GPR34, a G protein-coupled receptor, act on histamine-releasing activity on antigen- or concanavalin A-stimulated rat mast cells and synergize with nerve growth factor on rat mast cells. It is a lipid such as lysophosphatidylserine having the activity of releasing histamine. GPR34 antagonists can be used, for example, for histamine release inhibitors, immune diseases, edema, hyperacidity, etc.