Evaluation of vacuum energy density around geometrical defects by the WKB method

The energy density of a conformally invariant scalar field around a cosmic string is estimated by the WKB method. This approach reproduces the precise value obtained by the other authors using the exact mode-sum method or the method of mirror images. The approximation is also in good agreement with the exact result in the three-dimensional case. We also evaluate the energy density of a conformally invariant scalar field around a global monopole by the same way and find the known result again.