A tilt sensor comprises a multi-layer circuit board, a holder, a first light-sensing component, a second light-sensing component, a light-emitting component, and a movable component. The multi-layer circuit board comprises a first layer board and a second layer board disposed on the first layer board. The second layer board comprises first, second, and third holding spaces each of which exposing a respective portion of the first layer board. The holder is disposed on the second layer board and has a containing recess connecting the first, second, and third holding spaces. A bottom surface of the containing recess is reflective. The first and second light-sensing components and the light-emitting component are disposed on the first layer board and in the first, second, and third holding spaces, respectively. The light-emitting component emits a light beam toward the bottom surface of the containing recess. The movable component is disposed in the containing recess.