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We present an analysis of ALMA multi-band dust-continuum observations for 28spectroscopically-confirmed bright Lyman-break galaxies at $5<z<8$. Our sampleconsists of 11 galaxies at $z\sim6$ newly observed in our ALMA program, whichsubstantially increases the number of $5<z<8$ galaxies with both rest-frame 88and 158 $\mu{\rm m}$ continuum observations, allowing us to simultaneouslymeasure the IR luminosity and dust temperature for a statistical sample of$z\gtrsim5$ galaxies for the first time. We derive the relationship between theUV slope ($\beta_{\rm UV}$) and infrared excess (IRX) for the $z\sim6$galaxies, and find a shallower IRX-$\beta_{\rm UV}$ relation compared to theprevious results at $z\sim2$--4. Based on the IRX-$\beta_{\rm UV}$ relationconsistent with our results and the $\beta_{\rm UV}$-$M_{\rm UV}$ relationincluding fainter galaxies in the literature, we find a limited contribution ofthe dust-obscured star formation to the total SFR density, $\sim30\%$ at$z\sim6$. Our measurements of the dust temperature at $z\sim6-7$, $T_{\rmdust}=40.9_{-9.1}^{+10.0}\,{\rm K}$ on average, supports a gentle increase of$T_{\rm dust}$ from $z=0$ to $z\sim6$--7. Using an analytic model withparameters consistent with recent {\it{JWST}} results, we discuss that theobserved redshift evolution of the dust temperature can be reproduced by an$\sim0.6\,{\rm dex}$ increase in the gas depletion timescale and $\sim0.4\,{\rmdex}$ decrease of the metallicity. The variety of $T_{\rm dust}$ observed athigh redshifts can also be naturally explained by scatters around thestar-formation main sequence and average mass-metallicity relation, includingan extremely high dust temperature of $T_{\rm dust}>80\,{\rm K}$ observed in agalaxy at $z=8.3$.

SERENADE II: An ALMA Multi-Band Dust-Continuum Analysis of 28 Galaxies at $5