A lens device including a lens barrel, a lens, a linear reciprocating motion mechanism, and a power machine is provided. The lens is disposed in the lens barrel. The linear reciprocating motion mechanism includes a first movable part and a second movable part. The first movable part is coupled to the lens barrel. The second movable part is coupled to the first movable part. The power machine is coupled to the second movable part to drive the second movable part to rotate, thereby driving the first movable part, and the first movable part drives the lens barrel to perform a linear reciprocating motion, wherein when the second movable part is rotated by N turns, the first movable part is rotated by one turn, and N is greater than or equal to 10.