  We measured both the differential cross section ($\sigma_{p,p^\prime}$
$=d^2\sigma/d\Omega dE_{x}$) and the $\gamma$-ray emission probability
($R_\gamma(E_x)$ $=\sigma_{p,p^\prime\gamma}$/$\sigma_{p,p^\prime}$) from the
giant resonances excited by $\rm^{12}C$(\textit{p,p}$^\prime$) reaction at 392
MeV and 0$^\circ$, using a magnetic spectrometer and an array of NaI(Tl)
counters. The absolute value of $R_\gamma(E_x)$ was calibrated by using the
well-known $\gamma$-ray emission probability from $\rm^{12}C^* ( 15.11$ MeV, $
1^+$, $T=1$) and $\rm^{16}O^* ( 6.9$ MeV, $2^+$, $T=0$) states within 5\%
uncertainty. We found that $R_\gamma(E_x)$ starts from zero at $E_x=16$ MeV,
increases to a maximum of 53.3$\pm$0.4$\pm$3.9\% at $E_x=27$ MeV and then
decreases. We also compared the measured values of $R_\gamma(E_x)$ with
statistical model calculation based on the Hauser-Feshbach formalism in the
energy region $E_x=$ 16-32 MeV and discussed the features of $\gamma$-ray
emission probability quantitatively.
