In typical 
colliding experiments, the luminosity is measured by counting the number
of small angle Bhabha scattering events. This is a technique used under the assumption that
the collision energy is reasonably monochromatic, except for the effect of the initial state
radiation.
However, this is no longer a valid assumption in high energy linear colliders,
including JLC. This is because of the significant energy spread in collisions associated with
beamstrahlung energy loss. Rather, the differential luminosity with respect to the
effective center-of-mass energy (hereafter called ``luminosity spectrum") needs to be
measured. Miller et al. have proposed an idea for measuring the luminosity-spectrum by using
the acollinearity angle of the large angle Bhabha events[12]. The energy difference
between 
- and 
beams appears in the acollinearity angle of the Bhabha scattering.
The potentiality of this idea has been studied by means of a luminosity-spectrum generator and
a Bhabha event-generator.