In this paper, the analysis of interference in wireless terahertz band networks is presented. The estimation of the mean level of interference at the target receiver is performed, taking into account such inherent properties of the terahertz band systems as the use of highly-directional antennas, signal attenuation caused by molecular absorption, as well as signal blockage by the human body. Additionally, the comparison of the results for two simplified antenna radiation patterns is performed. The obtained results can be used as a building block for further performance evaluation of terahertz band wireless networks, including the estimation of signal-to-interference ratio and network capacity.