To obtain a focus position detecting device capable of accurately detecting a focus position regardless of the F value of a lens to be used.

When the F value of an image pickup optical system is larger than F2.8 (dark), the focus position is detected by using small-aperture lenses 1445a to 145c corresponding to F5.6 out of the image reforming lenses of 1st and 2nd image reforming lens arrays 145A and 145B and the 1st to the 3rd islands 146a to 146c of 1st and 2nd sensors corresponding to the lenses 145a to 145c. Meanwhile, when the F value of the image optical system 201 is smaller than F2.8 (bright), the focus position is detected by using a large-aperture lens 145d corresponding to F2.8 out of the image reforming lenses of the lens arrays 145A and 145B and the 4th island 146d of the 1st and the 2nd sensors corresponding to the lens 145d.