The key module for the mobile device includes a key top 6; a base plate 2 on which a fixed-side contact 7 is disposed; a dome 4 that is disposed on the fixed-side contact 7; a dome sheet 5 that covers the dome 4; an LED module 3 disposed on the base plate 2; and a light guide plate 1 that is disposed on the dome sheet 5. Such a configuration causes light emitted by the LED module 3 propagates within the light guide plate, and exits the light from an exit surface of the light guide plate on the key top 6 side. The light guide plate 1 is formed by inkjet printing a dot pattern in white ink onto a surface of a base material formed from a thermosetting polyurethane elastomer sheet, which surface is opposite to the exit surface of the light guide plate.