Thank you for help!
In any 3D software every face is actually made of triangles. If a face is too twisted, in Render mode you begin to see the triangles within. To avoid this artefact you can use the Auto-Smooth option, if your object is not smooth shaded (like in your case) it will kill the artefact within every face. In the case of a smooth shaded object it will also smooth the angles between faces, depending on the angle chosen.