  We deduce momentum sum rules for the parton structure functions of a photon
target. Non-perturbative QCD contribution to the momentum sum rules follows
from conservation of the energy--momentum tensor and it is calculated through
the hadronic part of the photon vacuum polarization operator. The contribution
of the unresolved photon unambiguously follows from gauge invariance,
renormalizability and asymptotic freedom in QCD. We also compare available
parametrizations of parton distribution in a photon with the deduced sum rules.
