The work describes a differential near-field scanning optical microscopy (DNSOM) mechanism, which represents a valuable tool for imaging with sub-wavelength resolution. In this approach the acquisition is carried out using squared apertures with dimensions comparable with the wavelength. The aperture, mounted on a xy translational stage driven with piezo-actuators, is scanned in the near field of the object of interest, illuminated by a laser source. In the measurements, a THz quantum cascade laser (QCL) laser is employed emitting at 2.9 THz.The resolution is presently limited by the numerical aperture of the light collection system and by the relative aperture-sample distance. Next steps consist in further improvements of the system in order to reach a resolution of ~ 1 mum and in the spectroscopic analysis of biological samples.