  A method is formulated for the description of lambda hypernuclei in the
framework of the unitary-model-operator approach (UMOA). The method is applied
to $_{\Lambda}^{17}$O. A lambda-nucleon effective interaction is derived,
taking the coupling of the sigma-nucleon channel into account. The lambda
single-particle energies are calculated for the 0s_{1/2}, 0p_{3/2} and 0p_{1/2}
states employing the Nijmegen soft-core (NSC), J\"ulich model-\~A (J\~A) and
model-\~B (J\~B) hyperon-nucleon potentials.
