PURPOSE: To reduce the number of parts and to simplify structure by providing different straight-advance moving members respectively meshing with 1st and 2nd screw shafts and individually straight-advance moved by each rotation.

CONSTITUTION: A lead screw 22 in parallel with the moving direction of both group lenses L2 and L3 and the straight-advance moving members 36 and 37 having a needle 41 meshing with the screw 22 are installed on a pedestal. The screw 22 is constituted of the 1st and the 2nd screw shafts 26 and 27. The member 36 allowing the needle 61 to mesh with the large-diameter screw 28 of the 1st screw shaft 26 moves forward or backward on an optical axis following the lead of the screw shaft 26. The member 37 allowing the needle 41 to mesh with the 2nd screw shaft 27 moves forward or backward on the optical axis following the lead of the screw shaft 27.