A light guiding device can be used for a reader/writer device (1) for a noncontact communication medium. The light guiding device includes a light guide plate (10) which includes: a transparent main body part (17) which can transmit light inputted from an LED (9); and a protruding part (18) which is provided so as to protrude from a part of an end surface of the transparent main body part (17). A diffusely reflecting part (19) is provided on a lower surface side of the transparent main body part (17), and a first light emitting part (7) is provided on an upper surface side of the transparent main body part (17). Reflecting parts (21) are provided on an upper surface side and a lower surface side of the protruding part (18), and a second light emitting part (8) is provided at a leading end of the protruding part (18). In the case where the first light emitting part (7) is covered by the noncontact communication medium, the second light emitting part (8) makes it possible to check a light emitting state. Accordingly, the light guiding device is provided, in which the light emitting state of the light emitting part can be easily checked.