Provided are agents comprising a phosphatidylcholine as an active ingredient to serve as a vein-formation promoting agent capable of promoting vein-like morphological change of tumor vessels, a vessel-diameter enlarging agent capable of enlarging the diameter of tumor vessels, a blood vessel-connection promoting agent capable of promoting connection of tumor vessels to each other without mediation of a lysophospholipid receptor, a leukocyte-infiltration promoting agent capable of promoting infiltration of leukocytes throughout a tumor region without mediation of a lysophospholipid receptor, and an antitumor immunostimulatory agent capable of promoting infiltration of leukocytes throughout a tumor region without mediation of a lysophospholipid receptor.
