The model is based on the assumption that logarithmic spiral arms of most spiral galaxies are due to the totating density wave\(\tilde v(r,{\text{ }}\theta {\text{, }}t)\) obeying the steady wave equation\(\nabla ^2 \tilde v(r,{\text{ }}\theta ) + (q^2 /r^2 )\tilde v(r,{\text{ }}\theta {\text{)}} = {\text{0}}\). It is shown that this equation accounts also for the so-called Titius-Bode's law in the solar system.