A lysophospholipid receptor-activating substance is able to normalize abnormal blood vessels in tumors without affecting normal blood vessels and to thus induce or promote infiltration of leukocytes into the whole or partial region of a tumor. Therefore, such a lysophospholipid receptor-activating substance is useful as a leukocyte infiltration promoter and an antitumor immunostimulant. Moreover, such a lysophospholipid receptor-activating substance can enhance cancer immunotherapy when used in combination with cancer immunotherapy, and is therefore useful as an agent for enhancing cancer immunotherapy.