A compound is provided which is effectively adsorbed to a substrate surface, such as an immune reaction vessel, has extremely high protein adsorption inhibitory effect that inhibits non-specific adsorption of protein or the like, and has excellent washability to retain the inhibitory effect before and after washing operation of the substrate. Also provided are a protein adsorption inhibitor using the compound, a phosphorylcholine-modified substrate, and a method for inhibiting protein adsorption. The compound is a phosphorylcholine group-containing polyethylene glycol derivative represented by formula (1). The method for inhibiting protein adsorption of the present invention includes the step of forming an adsorbed layer of the derivative on a substrate surface.