Conventional laser mapping sensor that adopts laser beam has been dangerous because an operator is exposed directly to laser beam, which comes out of the load port through FOUP. Panasonic has succeeded in developing LED to adopt as light source for M-DW1. Therefore an operator's safety is ensured. Nitride-coated wafers absorb light at certain wavelengths depending on the coating thickness. If the sensor uses the laser beam having a single wavelength, the beam may be absorbed completely, resulting in wafer detection error. The M-DW1 uses a LED light source with a wide wavelength band that allows to detect nitride-coated wafers successfully. The sensor responds in 0.5 ms, meeting the requirements of both high speed and high accuracy in wafer detection. The M-DW1, which detects wafers not by the light amount but by the light position, can detect the glass wafers regardless of the light amount.