A polyalkylene glycol derivative containing a compound of the formula (1): wherein R is a hydrocarbon group having 1 to 24 carbon atoms, OA2 is an oxyalkylene group having 2 to 4 carbon atoms, the groups represented by R are the same or different from each other in one molecule, the groups represented by OA2 are the same or different from each other in one molecule, m is an average number of moles of the oxyalkylene group added, m represents 10 to 1000, and X represents a functional group capable of chemically reactive with a bio-related substance, polydispersity Mw/Mn of the above polyalkylene glycol derivative in gel permeation chromatography satisfying the following relationship: Mw/Mn≦1.07 wherein Mw represents a weight average molecular weight and Mn represents a number average molecular weight.