The invention provides a highly sensitive measurement and detection technique for dioxins, which has advantages inherent to immunoassay; and markers to be used in the technique. Disclosed are a biotinylated dioxin derivative of formula (1): 1 (wherein X represents a hydrogen atom or a chlorine atom; R1 represents a biotin residue; R2's, which may be identical to or different from one another, independently and individually represent an arginine residue or a lysine residue; n is an integer from 1 to 5 inclusive; and m is an integer from 1 to 3 inclusive), and an immunoassay method for dioxins characterized by using the derivatives as a marker.