  We calculate the effective mass of a single quantized vortex in the BCS
superconductor at finite temperature. Based on effective action approach, we
arrive at the effective mass of a vortex as integral of the spectral function
$J(\omega)$ divided by $\omega^3$ over frequency. The spectral function is
given in terms of the quantum-mechanical transition elements of the gradient of
the Hamiltonian between two Bogoliubov-deGennes (BdG) eigenstates. Based on
self-consistent numerical diagonalization of the BdG equation we find that the
effective mass per unit length of vortex at zero temperature is of order $m
(k_f \xi_0)^2$ ($k_f$=Fermi momentum, $\xi_0$=coherence length), essentially
equaling the electron mass displaced within the coherence length from the
vortex core. Transitions between the core states are responsible for most of
the mass. The mass reaches a maximum value at $T\approx 0.5 T_c$ and decreases
continuously to zero at $T_c$.
