A first light detector and a second light detector, in each of which a plurality of light-receiving elements are arranged in an adjoining manner, are respectively provided anterior to an image-forming point of first reflected light and posterior to an image-forming point of second reflected light. A focal-point detecting circuit includes: maximum value selectors for selecting the maximum values from sums of light-receiving signals from adjoining light-receiving elements of the first light detector and the second light detector respectively; a total value operator for obtaining the total value of light-receiving signals from all the light-receiving elements of each of the first light detector and the second light detector; a light-detecting-signal operator for obtaining light-detecting signals by subtracting the maximum values from the total values; and an error-signal operational circuit for outputting a difference between the light-detecting signals to a servo circuit as a signal based on an amount of displacement between a focal point and a measuring face.