To provide a protein adsorption preventing agent having an excellent film forming property and excellent water solubility, capable of easily coating various substrates, and having an excellent protein adsorption preventing property, and also to provide a protein adsorption preventing film using the protein adsorption preventing agent and a medical tool including the protein adsorption preventing film. A protein adsorption preventing agent containing an aqueous solvent and a polymer having at least a structural unit (A) represented by the following formula (1) (wherein R1 represents a methyl group or an ethyl group and n represents an integer of 1 to 10), and further containing a hydrophobic group, a protein adsorption preventing film using the protein adsorption preventing agent, and a medical tool including the protein adsorption preventing film.
