Provided are a detection apparatus, a detection method and an optically transparent member, which can detect a decrease in a precision. A convolution is performed by a convolution portion 80 contained in an image processing portion 38, to acquire the distribution information indicating the light intensity distribution of a light beam, which is totally reflected at the interface and which is incident at a plurality of angles to an dielectric block 52 so as to be totally reflected at the interface of the dielectric block 52. A spatial frequency resolution is performed on the light intensity distribution indicated by the distribution information acquired, by a detection precision evaluating portion 86 contained in the image processing portion 38, to thereby derive the light intensity distribution of each spatial frequency of the light beam. The precision is detected by comparing the light intensity distribution derived, with a threshold value predetermined for each spatial frequency.