  We carried out $^{12}$CO($J$ = 1-0) observations of the Galactic gamma-ray
supernova remnant (SNR) Kesteven 79 using the Nobeyama Radio Observatory 45 m
radio telescope, which has an angular resolution of $\sim20$ arcsec. We
identified molecular and atomic gas interacting with Kesteven 79 whose radial
velocity is $\sim80$ km s$^{-1}$. The interacting molecular and atomic gases
show good spatial correspondence with the X-ray and radio shells, which have an
expanding motion with an expanding velocity of $\sim4$ km s$^{-1}$. The
molecular gas associated with the radio and X-ray peaks also exhibits a
high-intensity ratio of CO 3-2/1-0 $>$ 0.8, suggesting a kinematic temperature
of $\sim24$ K, owing to heating by the supernova shock. We determined the
kinematic distance to the SNR to be $\sim5.5$ kpc and the radius of the SNR to
be $\sim8$ pc. The average interstellar proton density inside of the SNR is
$\sim360$ cm$^{-3}$, of which atomic protons comprise only $\sim10$ $\%$.
Assuming a hadronic origin for the gamma-ray emission, the total cosmic-ray
proton energy above 1 GeV is estimated to be $\sim5 \times 10^{48}$ erg.
