An object of the present invention is to provide a luminescent ethylene-based copolymer which causes no problem of the shift or bleeding-out of a fluorescent body, which has a high workability, desired optical properties and a good light stability and which is easily handled in a kneading step; and a encapsulant composition for a solar cell, using this copolymer. Another object of the invention is to provide a encapsulant layer, for a solar cell, which is formed using the photovoltaic encapsulant composition to cause no problem of the shift or bleeding-out of a fluorescent body in this layer and to have desired optical properties and a good light stability; and a photoelectromotive module having this layer. The copolymer is a luminescent ethylene-based copolymer including, as a monomer component, a fluorescent dye compound having an unsaturated bond.