We have realized a differential Near-field Scanning Optical Microscope (NSOM) working with subwavelength resolution in the THz spectral region. The system employs a quantum cascade laser emitting at λ ∼ 105 μ as source, and the method, differently from conventional NSOM, involves diffracting apertures with size comparable to the wavelength. This concept ensures a higher signal-to-noise level at the expense of an additional computational step. In the implementation here reported λ/10 resolution has been achieved; present limiting factors are investigated through finite difference time domain simulations.