  We investigate physical reasons for high dust temperatures
($T_\mathrm{dust}\gtrsim 40$ K) observed in some high-redshift ($z>5$) galaxies
using analytic models. We consider two models that can be treated analytically:
the radiative transfer (RT) model, {where a broad distribution of values for
$T_\mathrm{dust}$ is considered}, and the one-tempearture (one-$T$) model,
which assumes {uniform $T_\mathrm{dust}$}. These two extremes {serve to bracket
the most realistic scenario}. We adopt the Kennicutt--Schmidt (KS) law to
relate stellar radiation field to gas surface density, and vary the dust-to-gas
ratio. As a consequence, our model is capable of predicting the relation
between the surface density of star formation rate ($\Sigma_\mathrm{SFR}$) or
dust mass ($\Sigma_\mathrm{dust}$) and $T_\mathrm{dust}$. We show that the high
$T_\mathrm{dust}$ observed at $z\gtrsim 5$ favour low dust-to-gas ratios
($\lesssim 10^{-3}$). An enhanced star formation compared with the KS law gives
an alternative explanation for the high $T_\mathrm{dust}$. The dust
temperatures are similar between the two (RT and one-$T$) models as long as we
use ALMA Bands 6--8. We also examine the relation among $\Sigma_\mathrm{SFR}$,
$\Sigma_\mathrm{dust}$ and $T_\mathrm{dust}$ without assuming the KS law, and
confirm the consistency with the actual observational data at $z>5$. In the
one-$T$ model, we also examine a clumpy dust distribution, which predicts lower
$T_\mathrm{dust}$ because of the leakage of stellar radiation. This enhances
the requirement of low dust abundance or high star formation efficiency to
explain the observed high $T_\mathrm{dust}$.
