A polyoxyethylene derivative represented by the formula (1): wherein a whole molecular weight of the polyoxyethylene derivative is 500 to 160,000; n is 5 to 3650; L1, L2, and L3 each independently represent an alkylene group, a phenylene group, an ester bond, an amide bond, an ether bond, a urethane bond, a carbonate bond, a secondary amino group, or a combination thereof; X represents a functional group capable of reacting with a bio-related substance; Y represents a hydrophilic group having plural hydroxyl groups made from a residual group of xylitol or volemitol or a residual group of polyglycerin of trimer to 31-mer; Z represents a residual group of a compound having 2 to 5 active hydrogen atoms; b and c are as follows: 1≦b≦4, 1≦c≦4, and 2≦b+c≦5; and d and e each independently are 0 or 1.