The present invention provides: a hyaluronic acid derivative which contains a disaccharide unit represented by formula (I) or disaccharide units respectively represented by formulae (I) and (II); and a complex which is composed of the hyaluronic acid derivative and a drug.La presente invención proporciona un derivado de ácido hialurónico que contiene una unidad de disacárido representada por la fórmula (I) o la fórmula (I) y (II) y un complejo que contiene el derivado del ácido hialurónico y un fármaco.