Position determining system

The present invention is directed to a position determining system (100) of reduced errors in measurements and determination of surfaces even if a rotary light source emitting laser beam rotates unevenly. The position determining system is comprised of a rotary light source rotating and emitting laser beams (b1, b2, b3), an encoder detecting a rotation position of the rotary light source, and a transfer means transmitting data detected by the encoder, and the system particularly comprises at least two rotary laser devices (151, 152) disposed separate from each other, a light sensor (154) having a light receiving unit that receives the laser beams emitted from the rotary laser devices, so that a position of the light sensor is determined depending upon the laser beams incident upon the light receiving unit and information transmitted by the transfer means.