An optical displacement measuring instrument, comprising: a light source; a collimating lens that collimates light from the light source and outputs the light as parallel light; an objective lens that condenses the parallel light from the collimating lens, illuminates the condensed light onto a measuring face of a target object and receives reflected light from the measuring face; a moving unit that moves the objective lens or a focusing lens disposed between the objective lens and the light source along an optical axis; a position detector that detects a position of the objective lens or a position of the focusing lens; and a focusing unit that recognizes a positional displacement between a focal point of the objective lens and the measuring face based on reflected light and operates the moving unit to bring the focal point of the objective lens into coincidence with the measuring face, the reflected light having been reflected on the measuring face and having passed though the objective lens, wherein the focusing unit includes: a light splitter for splitting the reflected light having reflected on the measuring face and having passed through the objective lens into two split light of first reflected light and second reflected light; a first light-receiving element array and a second light-receiving element array respectively disposed anterior to an image-forming point of the first reflected light and posterior to a image-forming point of the second reflected light, a plurality of pixels being two-dimensionally aligned in each of the first light-receiving element array and the second light-receiving element array; a light-receiving signal calculator that, among light-receiving signals from a plurality of pixels within an area predetermined in each of the first light-receiving element array and the second light-receiving element array, excepts light-receiving signals from pixels having the highest brightness to the nth highest brightness (n being an integer) and calculates a total value of light-receiving signals from the remaining pixels; and a servo circuit for operating the moving unit so that the total value of the light-receiving signals of the first light-receiving element array calculated by the light-receiving signal calculator becomes equal to the total value of the light-receiving signals of the second light-receiving element array calculated by the light-receiving signal calculator, the operation of the moving unit bringing the focal point of the objective lens into coincidence with the measuring face. a light source; a collimating lens that collimates light from the light source and outputs the light as parallel light; an objective lens that condenses the parallel light from the collimating lens, illuminates the condensed light onto a measuring face of a target object and receives reflected light from the measuring face; a moving unit that moves the objective lens or a focusing lens disposed between the objective lens and the light source along an optical axis; a position detector that detects a position of the objective lens or a position of the focusing lens; and a focusing unit that recognizes a positional displacement between a focal point of the objective lens and the measuring face based on reflected light and operates the moving unit to bring the focal point of the objective lens into coincidence with the measuring face, the reflected light having been reflected on the measuring face and having passed though the objective lens, wherein the focusing unit includes: a light splitter for splitting the reflected light having reflected on the measuring face and having passed through the objective lens into two split light of first reflected light and second reflected light; a first light-receiving element array and a second light-receiving element array respectively disposed anterior to an image-forming point of the first reflected light and posterior to a image-forming point of the second reflected light, a plurality of pixels being two-dimensionally aligned in each of the first light-receiving element array and the second light-receiving element array; a light-receiving signal calculator that, among light-receiving signals from a plurality of pixels within an area predetermined in each of the first light-receiving element array and the second light-receiving element array, excepts light-receiving signals from pixels having the highest brightness to the nth highest brightness (n being an integer) and calculates a total value of light-receiving signals from the remaining pixels; and a servo circuit for operating the moving unit so that the total value of the light-receiving signals of the first light-receiving element array calculated by the light-receiving signal calculator becomes equal to the total value of the light-receiving signals of the second light-receiving element array calculated by the light-receiving signal calculator, the operation of the moving unit bringing the focal point of the objective lens into coincidence with the measuring face.