The purpose of the invention is to provide a tumor sealant to be used for therapy that enables curative treatment, by improving an extremely simple, minimally-invasive therapy consisting of starving tumor cells without radiation exposure or causing serious side effects by drugs. The invention provides a tumor sealant that utilizes the EPR effect specific to tumor cells, that covers the tumor cells of each tissue by particles that permeate only tumor blood vessels and accumulate and settle in the tumor, that does not release angiogenesis-inducing factors, that does not separate tumor cells having improved mobility in a hypoxic state and move them to another location, that permeates gaps of from 50 nm to 500 nm of the tumor vascular endothelial cells, and that biodegrades after adhering to the tumor cells and extracellular matrix near the gaps.
