A focus-detecting method and device is provided for detecting when an object is arranged in a focal plane of an object lens. A pattern or a light-source image is formed based on a light-beam on the object through an object blends. The light-beam emitted from the light-beam pattern on the object is reflected from the object to the object lens. Plural images of the light-beam pattern are formed along an optical axis of the object lens based on the light-beam passed through the object lens. A conjugate image point of the pupil of the object lens is formed between the object lens and a final imaging plane based on the light-beam passed through the object lens. The final light-beam pattern is imaged toward the final imaging plane. A photodetecting device is arranged on the final imaging plane. A shielding plate is provided to partially block the light beam from reaching the photodetecting device. The shielding plate is placed substantially at the conjugate image point of the pupil of the object lens.