  The spectral characteristics of the pulsed gamma-ray emission from
outer-magnetospheric particle accelerators are investigated. Either positrons
or electrons are accelerated outwards by the magnetic-field-aligned electric
field to emit gamma-rays via curvature process. Since the particles move along
relatively straight paths in the trailing side of a rotating magnetosphere,
they attain higher Lorentz factors to emit more energetic gamma-rays than those
in the leading side. It is first demonstrated that the cutoff energy of the
curvature radiation evolves with the rotation phase owing to the variation of
the curvature radii of the particle paths and maximizes at a slightly later
phase of the trailing peak in the gamma-ray light curve.
