A focus detector for detecting a defocus amount of an image pickup optical system based on a displacement amount between two images formed by two beams divided from the image pickup optical system and passed through two pupil areas, the focus detector including: two lenses; two phase difference sensors for photoelectrically converting two subject images formed by the two lenses into two image signals; a correlation computing unit for dividing the two image signals based on a reference number-of-pixels to compute an image displacement amount for each divided area; a waveform coincidence computing unit for computing a coincidence degree of the two image signals in the each divided area in which the image displacement amount is computed by the correlation computing unit; and a defocus computing unit for computing a defocus amount based on the coincidence degree of the two image signals computed by the waveform coincidence computing unit.