A highly sensitive and inexpensive spatial optical transmission system using an LD (Laser Diode) as a light source and making use of the characteristics of a light receiving element is provided. The system includes a laser chip oscillating a laser beam, and a light receiving element receiving and converting to an electric signal the laser beam emitted from the laser chip and propagated through a space. The laser chip oscillates a laser beam having a wavelength of which absorption coefficient at the light receiving element is in a range of 0.001 to 0.3 m^-1.