  Observations of gravitational lenses in strong gravitational fields give us a
clue to understanding dark compact objects. In this paper, we extend a method
to obtain a deflection angle in a strong deflection limit provided by Bozza
[Phys. Rev. D 66, 103001 (2002)] to apply to ultrastatic spacetimes. We also
discuss on the order of an error term in the deflection angle. Using the
improved method, we consider gravitational lensing by an Ellis wormhole, which
is an ultrastatic wormhole of the Morris-Thorne class.
