The paper presents principles of a simple method, which in two stages makes possible the approximate calculations of statically indeterminate truss systems. The proposed two-stage method applies rules of other methods used for calculations of the statically determinate trusses. In each of the both stages there are considered the statically determinate trusses, patterns of which are obtained as results of suitable taking selected members out from pattern of the basic statically indeterminate truss. These intermediate trusses have the same clear span and construction depth like the basic indeterminate truss but they are loaded by forces of the half values applied to nodes of the same positions like in the basic one. The proposed two-stage method uses theorems and features of calculus of vectors as well as principle of the superposition method. Final values of forces acting in particular members of the basic statically indeterminate truss are resultants of forces calculated in each stage in the counterpart members of the statically determinate trusses. There are presented results of calculations carried out for two cases of loading of a selected type of the plane truss. These results are compared with results of forces determined for the same truss by application of computer calculations carried out by method appropriate for the statically indeterminate systems.