An optical ranging sensor includes a light emitting unit for projecting a light beam on an object to be measured, a light receiving unit on which a light spot of reflected light of the light beam from the object is formed, and a processing circuit unit for processing output signals from the light receiving unit and detecting a distance to the object. The light receiving unit includes an effective light receiving part having light receiving cells arranged in matrix form in a first direction in which a position of the light spot moves as the object moves along a direction of an optical axis of the light emitting unit, and in a second direction orthogonal to the first direction. A size of the effective light receiving part in the second direction is not smaller than a radius of the light spot but not larger than a diameter thereof.