Provided are a measuring apparatus and a plasma processing apparatus. The measuring apparatus includes a diffraction grating which dispersed light reflected by one surface of an examination target having a thickness D and light reflected by a rear surface of the examination target, as incident light, a photodiode array (1080 in which a plurality of photo-detection elements receiving light dispersed by the wavelength dispersion device and detecting a power of the received light in are provided in an array shape, and a piezoelectric device which is attached to the detector to convert an applied voltage into a mechanical power, wherein the detector detects the power of the received light when the detector is shifted by the mechanical power converted by the piezoelectric device as much as d/m, where d is a width of each of the photodetection elements in an array direction and m is an integer equal to or greater than 2.