Provided is a cell adhesion inhibitor which exhibits low cytotoxicity and an excellent cell adhesion prevention effect; a tool and an apparatus each having a surface modified through application of the cell adhesion inhibitor thereto; a method for producing each of the surface-modified tool and apparatus; a biomedical structure and a production method therefor; and a microchannel device and a production method therefor.The invention provides a cell adhesion inhibitor comprising, as an active ingredient, a polymer comprising a repeating unit having a sulfinyl group in a side chain thereof.