A lens unit and a rear-side focus adjustment system for infrared cameras capable of performing rear-side focus adjustments even when the position of the detection surface of an infrared sensor is unknown in an infrared camera. The lens unit is detachably mounted on a camera body that detects infrared light with an infrared sensor and converts the infrared light to an image signal, the lens unit including a focus adjustment lens and controller. The controller calculates a rear-side focus adjustment amount for causing a rear-side focus position to correspond to the detection surface based on an error between an actually measured focused position of the focus adjustment lens relative to an object at a particular object distance and a theoretically focused position of the focus adjustment lens calculated based on the object distance and controls the position of the focus adjustment lens based on the rear-side focus adjustment amount.