A novel method of terahertz (THz) microscopy was proposed for imaging of biological tissues with sub-wavelength spatial resolution. It allows for overcoming the Abbe diffraction limit and provides a sub-wavelength resolution thanks to the solid immersion effect – i.e. to the reduction in the dimensions of electromagnetic beam caustic, when the beam is focused in free space, at a small distance (smaller than the wavelength) behind the medium featuring high refractive index. An experimental setup realizing the proposed method was developed. It uses a backward wave oscillator, as a THz-wave emitter, and a Golay cell, as a THz-wave detector. In this setup, the radiation is focused behind the silicon hemisphere in order to realize the solid immersion effect. The spatial resolution of 0.15λ was demonstrated for the developed microscope, while the measurements were carried out at the wavelength of λ=500 μm, with the metal-air interface as a test object. Such a high spatial resolution represents a significant advantage over that of the previously reported arrangements of solid immersion microscopes. The solid immersion microscopy does not imply using any diaphragms or other near-field probes for achieving the sub-wavelength spatial resolution; thus, it eliminates the energy losses associated with such elements. The proposed methods were applied for imaging of biological tissues, and the observed results highlight its potential in biology and medicine.