Organic EL light source

A translucent material substrate is shaped such that high luminous efficiency can be obtained in an organic EL light source in which light is emitted from a translucent material substrate to the atmosphere. The organic EL light source includes a luminescent layer composed of an organic luminescent element, a transparent electrode and a counter electrode, and a translucent material substrate. The translucent material substrate has a truncated quadrangular pyramidal shape having a surface in abutment with the luminescent layer as the bottom face of the shape. The bottom face is formed in the shape of a square in which the length of one side is “b”, and the top face opposite the bottom face is formed in the shape of a square in which the length of one side with respect to the same extending direction as that of the “b”-length side is “a”. In addition, the shape of the translucent material substrate has thickness “c”. The translucent material substrate is shaped so that a ratio (a/b) of length a of a side of the top face to length b of a side of the bottom face in the same extending direction as that of the side is 0.3 or higher but not higher than 0.95, and that a ratio (b/c) of length b of a side of the bottom face to the thickness c of the translucent material substrate is 10 or lower.