30 is screwed and a second groove portion 73 to which the second protrusion 61 passing through the second guide slot 32 of the guide ring 30 is screwed thus the second cylinder 70 movably supports the first zoom ring 20 and the first cylinder 60 and guides a movement of the first zoom ring 20 and the first cylinder 60 the first groove portion 72 comprises a first inclination portion 72 a that is inclined from a boundary 78 of one end of the second cylinder 70 to a boundary 79 of another end thereof and extends in the circumferential direction and a second inclination portion 72 b that is inclined from an end portion of the first inclination portion 72 a to the boundary 78 and extends in the circumferential direction the first inclination portion 72 a performs a function of moving the first zoom ring 20 forward in the axial direction with respect to the second cylinder 70 the second inclination portion 72 b performs a function of moving the first zoom ring 20 backward in the axial direction with respect to the second cylinder 70 a third groove portion 82 is formed in an inner wall surface of the external cylinder 80 the second cylinder 70 is disposed in the external cylinder 80 and the fourth protrusion 71 of the second cylinder 70 is screwed to the third groove portion 82 so that the external cylinder 80 rotatably supports the second cylinder 70 because the fourth protrusion 71 is.