An AD converter is provided. The AD converter includes a light source unit which provides an optical signal, at least one waveguide unit which consists of a photonic crystal and transmits the optical signal, a modulation unit which applies a RF signal to the at least one waveguide unit, thereby modulating an optical signal output from the at least one waveguide unit, and a digital conversion unit which converts the modulated optical signal to a digital signal. Accordingly, a compact-sized AD convert can be realized.