DETECTOR OF FOCUS ERROR

PURPOSE:To obtain a focus error signal by constituting a photodetecor with two pairs of light detecting elements which receive exit lights which pass the outside and the inside of boundary planes of a pair of lens regions and operating detection signal values of these light detecting elements. CONSTITUTION:The photodetector consists of the first and the second light detecting elements 8B and 8A, which receive exist light which pass the outside and the inside of the boundary plane of the first lens region 30a out of a pair of lens regions 30a and 30b, and the third and the fourth light detecting elements 8D and 8C which receive exit lights which pass the outside and the inside of the boundary plane of the second lens region 30b. The focus error signal is obtained by the difference between the sum of the first and the fourth light detection signals from the first and the fourth light detecting elements 8B and 8C and the sum of the second and the third light detection signals from the second and the third light detecting elements.