  We study the emission from an old supernova remnant (SNR) with an age of
around 10^5 yrs and that from a giant molecular cloud (GMC) encountered by the
SNR. When the SNR age is around 10^5 yrs, proton acceleration is efficient
enough to emit TeV gamma-rays both at the shock of the SNR and that in the GMC.
The maximum energy of primarily accelerated electrons is so small that TeV
gamma-rays and X-rays are dominated by hadronic processes, pi^0-decay and
synchrotron radiation from secondary electrons, respectively. However, if the
SNR is older than several 10^5 yrs, there are few high-energy particles
emitting TeV gamma-rays because of the energy loss effect and/or the wave
damping effect occurring at low-velocity isothermal shocks. For old SNRs or
SNR-GMC interacting systems capable of generating TeV gamma-ray emitting
particles, we calculated the ratio of TeV gamma-ray (1-10 TeV) to X-ray (2-10
keV) energy flux and found that it can be more than ~10^2. Such a source
showing large flux ratio may be a possible origin of recently discovered
unidentified TeV sources.
