A spherical aberration correction mechanism 30 includes a lens holder 35 that holds a collimator lens 16, a main shaft 36a that supports the lens holder 35 in a linearly movable manner, a lead screw 34 supported in a rotatable manner and provided with a screw thread formed of a first slope and a second slope each in a spiral shape on an outer circumference thereof, and a stepping motor 13 that rotates the lead screw 34. The lens holder 35 includes a rack portion 41 disposed between the main shaft 36a and the lead screw 34 and having a slope that abuts on the first slope and first and second stoppers 46a and 46b that regulate a relative position of the rack portion 41 with respect to the lead screw 34 so that the rack portion 41 does not abut on the second slope.