The invention concerns a timepiece (1), notably a diver's watch, comprising a case (2) provided with a case middle (3), a light source (6), an element (4) movable between several positions relative to the case (2), and movable means for assembling the movable element (4) on the middle part (3) of the case (2). More particularly, the movable element (4) includes a light guide (10) and an upper cover (9) comprising an index (5), the guide (10) being configured to receive a light beam (8) from the light source through an entry face (12), and to guide light rays (18) through an exit face (15) of the light guide (10) in order to illuminate the index (5), whatever the position of the movable element (4) relative to the case (2).