  $^{56}$Ni is an important indicator of the supernova explosions, which
characterizes light curves. Nevertheless, rather than $^{56}$Ni, the explosion
energy has often been paid attention from the explosion mechanism community,
since it is easier to estimate from numerical data than the amount of
$^{56}$Ni. The final explosion energy, however, is difficult to estimate by
detailed numerical simulations because current simulations cannot reach typical
timescale of saturation of explosion energy. Instead, the amount of $^{56}$Ni
converges within a short timescale so that it would be a better probe of the
explosion mechanism. We investigated the amount of $^{56}$Ni synthesized by
explosive nucleosynthesis in supernova ejecta by means of numerical simulations
and an analytic model. For numerical simulations, we employ Lagrangian
hydrodynamics code in which neutrino heating and cooling terms are taken into
account by light-bulb approximation. Initial conditions are taken from Woosley
& Hegel (2007), which have 12, 15, 20, and 25 $M_\odot$ in zero age main
sequence. We additionally develop an analytic model, which gives a reasonable
estimate of the amount of $^{56}$Ni. We found that, in order to produce enough
amount of $^{56}$Ni, $\mathcal{O}(1)$ Bethe s$^{-1}$ of growth rate of the
explosion energy is needed, which is much larger than that found in recent
exploding simulations, typically $\mathcal{O}(0.1)$ Bethe s$^{-1}$.
