In this paper, we perform chiral perturbative analysis of an approximate lepton number symmetry associated with a sufficiently light neutrino in the type-I seesaw mechanism. For the Dirac mass matrix mD=(A,B,C), linearly independent components of C from A and B are treated as symmetry-breaking parameters. A deviation in the eigenvector of the massless mode δu occurs in the first-order perturbation and the lightest mass m1or 3∝detmD2/M3 emerges in the second-order. By solving the perturbation theory, we obtained specific expressions of δu and m1or 3. As a result, two complex parameters in mD are bounded to some extent from the eigenvector. These constraints are associated with the chiral symmetry and are not susceptible to renormalization.