Provided is an electronic device for detecting the relative rotational positions of two casing members optically without utilizing the difference in the reflecting efficiency between a plurality of reflecting portions. The electronic device comprises a first light emitting unit (51) for emitting a light (R), a first casing member (2) mounting the first light emitting unit (51), a first light receiving unit (61) for receiving the light (R), a second casing member (3) mounting the first light receiving unit (61) and so connected to the first casing member (2) as to rotate on a spindle (41), and a detecting unit for detecting the rotational position of the second casing member (3) relative to the first casing member (2), on the basis of the reception of the light (R) emitted from the first light emitting unit (51) in the first light receiving unit (61). The first light emitting unit (51) is arranged in the first casing member (2) and on a first circumference (C1) centering on the spindle (41), and the first light receiving unit (61) is arranged in the second casing member (3) and on a second circumference (C2) facing the first circumference (C1) and centering on the spindle (41).