An apparatus for measuring a semiconductor device is provided. The apparatus includes a beam emitter configured to irradiate an electron beam onto a sample having the entire region composed of a critical dimension (CD) region, which is formed by etching or development, and a normal region connected to the CD region, and an analyzer electrically connected to the beam emitter, and configured to select and set a wavelength range of a region in which a difference in reflectance between the CD region and the normal region occurs, after obtaining reflectance from the electron beam reflected by a surface of the sample according to the wavelength of the electron beam. A method of measuring a semiconductor device using the measuring apparatus is also provided. Therefore, it is possible to minimize a change in reflectance due to the thickness and properties of the semiconductor device, and set a wavelength range to monitor a specific wavelength, thereby accurately measuring and analyzing a CD value of a measurement part of the semiconductor device.