  We theoretically investigate strong-coupling effects on specific heat at
constant volume $C_{\rm V}$ in a superfluid Fermi gas with a tunable
interaction associated with Feshbach resonance. Including fluctuations of the
superfluid order parameter within the strong-coupling theory developed by
Nozi\`eres and Schmitt-Rink, we calculate the temperature dependence of $C_{\rm
V}$ at the unitarity limit in the superfluid phase. We show that, in the low
temperature region, $T^3$-behavior is shown in the temperature dependence of
$C_{\rm V}$. This result indicates that the low-lying excitations are dominated
by the gapless Goldstone mode, associated with the phase fluctuations of the
superfluid order parameter. Since the Goldstone mode is one of the most
fundamental phenomena in the Fermionic superfluidity, our results are useful
for further understanding how the pairing fluctuations affects physical
properties in the BCS-BEC crossover physics below the superfluid transition
temperature.
