An organic electroluminescent (EL) device which uses, as a luminescence-assisting agent, a novel coumarin derivative with a specific structure. The coumarin derivative effectively accelerates in such an organic EL device the transfer of excited energy in a host compound to a guest compound. Thus, the coumarin derivative affords the organic EL device which is superior in a color purity, luminescent efficiency and life expectancy when used in combination with appropriate host and guest compounds.