a light source means for generating an inspection light containing multiple lights modulated at a given number of different frequencies,

a light-receiving means for receiving the light generated at said light source means and passed through an object to be examined and for outputting synthesized signals made of a given number of electric signals each having a different frequency and an intensity corresponding to the received light, and

a detection means for detecting a signal with the same frequency as a frequency of a reference signal having a frequency corresponding to frequencies of modulated multiple lights in the synthesized signals, wherein

said detection means comprises:

an analog-digital conversion means for outputting digitized synthesized signal data by converting said synthesized signals having different frequencies to a digital signal,

a storage means for storing digitized data of a number of reference signals, where the number of reference signals is equal to the number of the synthesized signals output from the light-receiving means,

a delaying means for delaying said reference signals data read out from the storage means based on phase differences between said synthesized signals data output from said analog-digital conversion means and said reference signals data read out from the storage means,

a digital multiplication means for multiplying said synthesized signals data output from said analog-digital conversion means by said reference signals data delayed by the delaying means and for outputting the product of multiplication, and

a digital band-limitation means for taking out DC data from the output from said digital multiplication means.