Minimal $R$ operation in the $1/N$ expansion of $\sigma$ models

The renormalization structure of the $1/N$ expansion in $\sigma$ models is investigated. It is shown that the theory is renormalized by an $R$ operation in which subtractions are made only for some of the subgraphs with non-negative degree (of divergence). In contrast to the standard renormalization, such a renormalization manifestly preserves the nonlinear symmetry of the $\sigma$ model.