An optical integrated device includes a light source; a light splitting-and-guiding section that splits a reflected light beam into two end light beams, a connection light beam, and a residual light beam, and guides the two end light beams and the connection light beam in directions different from a direction of the residual light beam; and a light receiving section that receives the two end light beams and the connection light beam with photodetection devices divided, in the tangential direction, into at least two regions within a range in which the connection light beam is incident, receives the residual light beam with photodetection devices divided, in the tangential direction, into regions having widths corresponding to portions on which the two end light beams are incident, and outputs a detection signal in accordance with an amount of light received with each of the photodetection devices.