PURPOSE: To provide an optical element for auto-focusing which is inexpensive and reduces the height in the vertical direction of an object to be focused.

CONSTITUTION: Light L emitted from a light emitting element 1 is obiquely reflected downward by a second face (concave mirror) 12 and passes an exit face 13a of a third face 13 while being condensed in the horizontal direction and is reflected by a rear face (mirror face) 21a of a magneto-optical disk 21. Reflected light L1 is made incident on an optical element 10 from an incidence face 13b of the third face 13 while being condensed furthermore and is transmitted through a first face 11 and reaches a light reception face 3 of a light receiving element 2. In the in-focus state, the reflected light L1 is made incident on only a center part 2a of the light receiving element 2 but is not made incident on a side part 2b. In the out-of-focus state, the reflected light L1 is made incident on the center part 2a as well as the side part 2b of the light receiving element 2.