A substance that activates the lysophospholipid receptors is capable of normalizing abnormal blood vessels inside a tumor without affecting normal blood vessels, thereby causing or promoting the infiltration of leukocytes into some or all of the tumor region, and therefore is useful as a leukocyte infiltration promoter and a tumor immunoactivator. Furthermore, a substance that activates the lysophospholipid receptors can be used in combination with cancer immunotherapy to potentiate cancer immunotherapy, and therefore is useful as a cancer immunotherapy potentiator.