To provide a focus-detecting system capable of carrying out focus detection, even when infrared light region is not included in the light for a subject image.

A control part 10 compares the volume of the infrared light Lir, detected by a first focus detecting part 6, with a predetermined light volume standard value. When the volume of the infrared light Lir, detected by the first focus detecting part 6, is the standard value or smaller, and the focus detection by the infrared light Lir cannot be conducted, hologram function of a liquid crystal hologram 8 is set to on state, and focus detection is carried out with the use of a light L2 diffracted by the liquid crystal hologram 8. When the volume of the detected infrared light Lir is larger than the standard value and the focus detection can be made, the hologram function of the liquid crystal hologram 8 is set to off state, and focus detection is carried out with the use of the infrared light Lir that is reflected by a half mirror 5.

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