The lens device is capable of preventing a guide member from being disengaged from a guide groove with a simple construction, in which a projecting thread portion is formed on a sidewall of the guide groove for guiding a lens barrel in a straight motion and a recessed thread portion to be engaged with the projecting thread portion is formed on a sidewall of the guide member that is guided by the guide groove. A straight guide groove along the optical axis is formed on an inner peripheral surface of a first lens barrel, and a straight guide projection that is guided by the straight guide groove is formed on a second lens barrel. On a sidewall of the straight guide groove, a projecting thread portion is formed and on a sidewall of the straight guide projection, a recessed thread portion to be engaged with the projecting thread portion is formed.