  We calculate the number density and the energy density of a massive scalar
particle conformally coupled to gravity produced by gravitational particle
creation in the case the kination stage follows inflation. In this model, mode
functions are derived in terms of exact or numerical solutions of the equation
of motion without using the adiabatic approximation. We define the adiabatic
vacuum in each stage and calculate the produced number density and the energy
density. The resultant power spectra show that even the superheavy particle
which is as heavy as or much heavier than the Hubble scale during inflation can
be produced abundantly if the transition time scale from inflation to the
kination stage is smaller than the inverse of the mass of the scalar field. We
also give simple forms of the reheating temperature for two cases, one that the
produced scalar particles decay instantly and the other that they decay when
their energy density exceeds that of inflaton.
