According to the Jaeger’s theory, the minimum possible rock mass shear strength as a discontinuum actually corresponds to the shear strength of rock joints. Since failures in rock masses due to loads caused by civil structure and/or civil works occur mainly by exceeding their shear strength, the shear strength of rock joints has huge practical significance in rock engineering. For this reason, in this paper it was decided to analyse one of the factors which have very important influence on the shear strength of rock joints. Namely, natural rock joints are often filled with soft soil material and this infill material may have a significant and often decisive influence on the shear strength of rock joints. As a basis for the conducted analyses were used the results of direct shear tests under constant normal stress which was performed on natural or artificial specimens with horizontal infilled joints by various researchers around the word. Analyses have shown that some basic principles of mechanical behaviour of infilled rock joints during shearing can be reached. The thickness t and mechanical characteristic of the infill material have decisive influence on the peak and residual shear strength of infilled rock joints.