An electroluminescent device comprising electrodes consisting of a pair of an anode and a cathode, at least one of the electrodes being either transparent or translucent, and an organic layer interposed between said electrodes, wherein said organic layer comprises a copolymer comprising in the main chain thereof 9-oxo-9-phosphafluorene-2,7-diyl units and arylene units, represented by formula (1) below, and the thickness of said organic layer is from 1 nm to 10 m: ##STR1##

wherein R represents hydrogen, alkyl, cycloalkyl, aralkyl, aryl, alkyloxy, cycloalkyloxy, aralkyloxy, or aryloxy; Ar represents arylene; and n is an integer of 3 to 30,000.