An optical device includes a sub-mount 2 mounting first and second semiconductor lasers 1a, 1b and having an onboard part for the semiconductor lasers and an optical-path conversion mirror 7 integrated with each other, and a light receiving element 11 arranged on a light receiving element substrate to have first and second light receiving regions separated from each other by at least one parting line. In the optical device, one light gravity center of an optical spot, which is formed on the light receiving element 11 by homeward flux of light emitted from the first semiconductor laser 1a and reflected by an optical disc 55, and another light gravity center of an optical spot, which is formed on the light receiving element 11 by homeward flux of light emitted from the second semiconductor laser 1b and reflected by the optical disc 55 are together positioned on the parting line.