2D photonic crystals formed inside monocrystalline diamond to operate in the IR spectral range are reported. The photonic structures consisting of 150-μm-long graphitized wires arranged in a square matrix with a period of 4 μm were produced by laser writing with ultrashort pulses. Transmittance spectra (λ=1-14 μm) measured for the structures with increasing thickness demonstrate the occurrence of few minima being different for TM and TE polarization modes. Complex refraction index of the laser-modified material was evaluated for the first time in order to be used in computer simulation of the structures.