  We propose a new mechanism for the paramagnetic metal-insulator transition in
the layered perovskite Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ (0<x<0.2). The LDA+U
approach including spin-orbit coupling is used to calculate the electronic
structures. In Ca$_{2}$RuO$_{4}$, we show that the spin-orbit effect is
strongly enhanced by the Coulomb repulsion, which leads to an insulating phase.
When Ca is substituted by Sr, the effective spin-orbit splitting is reduced due
to the increasing bandwidth of the degenerate $d_{xz}$ and $d_{yz}$ orbitals.
For x=0.2, the compound is found to be metallic. We show that these results are
in good agreement with the experimental phase diagram.
