  We investigate the deflection angle in a strong deflection limit for a
marginally unstable photon sphere in a general asymptotically flat, static and
spherically symmetric spacetime under some assumptions to calculate
observables. The deflection angle of a light ray reflected by the marginally
unstable photon sphere diverges nonlogarithmically while the one reflected by a
photon sphere diverges logarithmically. We apply our formula to a
Reissner-Nordstrom spacetime and Hayward spacetime.
