To provide a planar illuminant easy to increase its area, with excellent heat resistance and suitable for a high-rigidity display device or signboard without causing camber due to moisture absorption or the like.

This planar illuminant is formed of a cyclic olefin resin of a random copolymer having a copolymerization composition containing, as monomer constituents, 10-40 mol% of at least 2-4C straight chain or branch α-olefin and 60-90 mol% of cyclic olefin expressed by the following general formula (1). In the formula, n=0, 1 or 2, R1-R12 are each independently hydrogen, a halogen atom or a hydrocarbon group, R9-R12 may form a monocycle or polycycle by being bonded to each other, and the monocycle or polycycle may have a double bond. The hydrocarbon group may be substituted by a halogen atom.