A switch illuminating EL sheet (7) has a structure such that a transparent protection film (8), a transparent electrode layer (9), a light emitting layer (10), a dielectric layer (11) and a back electrode layer (12) are layered in order from a light emitting face side. The transparent protection film (8) has a thickness of 10 .mu.m to 60 .mu.m. Further, the transparent electrode layer (9) is constituted of a conductive polymer. Such an EL sheet (7) can suppress wire breakage or failure to light due to keystroke stress or the like without impairing reliability and clicking feeling of a key switch. The EL sheet (7) is arranged, for example, between a key top (1) and a switch mechanism portion (2) as a light source for illuminating the key top portion (1).