Medical instrument includes a substrate, in which cells are in contact with or held on a surface of the substrate, and at least the surface of the substrate which holds the cells is formed of a fluorine-containing cyclic olefin polymer which contains a repeating structure unit represented by Formula (1), wherein in Formula (1), at least one of R1 to R4 is fluorine, an alkyl with 1 to 10 carbon atoms which contains fluorine, an alkoxy with 1 to 10 carbon atoms which contains fluorine, or an alkoxyalkyl with 2 to 10 carbon atoms which contains fluorine, R1 to R4 are selected from hydrogen and certain non-fluorinated groups when R1 to R4 do not contain fluorine, R1 to R4 may be the same as or different from each other, and R1 to R4 may be bonded to each other to form a cyclic structure.