  We have found that body-centered cubic (bcc) HfMoNbTiZr is a type-II BCS
high-entropy alloy (HEA) superconductor with a superconducting critical
temperature of $T_\mathrm{c}$=4.1 K. By employing a Debye temperature
$\theta_\mathrm{D}$ of 263 K and $T_\mathrm{c}$, the electron-phonon coupling
constant $\lambda_\mathrm{e-p}$ is calculated to be 0.63. The electronic
structure calculation revealed band broadening with energy uncertainties due to
atomic disorders. The superconducting properties are compared among equiatomic
quinary bcc HEA superconductors. $T_\mathrm{c}$ decreases with decreasing
$\lambda_\mathrm{e-p}$, which is negatively correlated with $\theta_{D}$. The
negative correlation between $\lambda_\mathrm{e-p}$ and $\theta_{D}$ would be
caused by a shorter phonon lifetime at a higher $\theta_{D}$, which is based on
phonon broadening due to atomic disorder and the uncertainty principle. The
Vickers microhardness was measured for several bcc HEA superconductors.
$T_\mathrm{c}$ is exceptionally low in a superconductor with relatively high
hardness, because the hardness generally increases with increasing
$\theta_\mathrm{D}$, and $\theta_\mathrm{D}$ is negatively correlated with
$T_\mathrm{c}$ in the high-entropy state.
