A rotationally extendable structure of a lens barrel includes a stationary ring; a rotational ring, supported by the stationary ring to move in an optical axis direction while rotating and includes a peripheral gear at the rear end thereof; and a rotational-ring drive gear. The rotational-ring drive gear includes a long gear portion, formed long enough to mesh with the peripheral gear over an entire moving range of the rotational ring, and a large-diameter gear portion at the rear end of the long gear portion. The rotational ring is provided in a rear end thereof with a large-diameter-gear accommodating recess. When the rotational ring is moved to a rearward movement extremity in the optical axis direction, the large-diameter gear portion enters into the large-diameter-gear accommodating recess so that the large-diameter gear portion coincides with the peripheral gear with respect to the optical axis direction.