The shell-model calculations have been performed for the E2 decay rate of He*Λ7. It is shown that the inclusion of nucleon-nucleon correlation in the usual independent-particle shell-model (IPSM) charge-density distribution of the core nucleus He6 provides an adequte description and leads to an identification of He*Λ7 as an isomeric state. It also provides a method of studying the effect of core polarization. The treatment of the core nucleus can be applied with confidence to other nuclear systems. It would be particularly useful for those nuclei which cannot be well described by simple IPSM wave functions-e.g., Li6.