  Recently, gamma-ray telescopes AGILE and Fermi observed several middle-aged
supernova remnants (SNRs) interacting with molecular clouds. It is likely that
their gamma rays arise from the decay of neutral pions produced by the
inelastic collision between cosmic rays (CRs) and nucleons, which suggests that
SNRs make the bulk of Galactic CRs. In this letter, we provide the analytical
solution of the distribution of CRs that have escaped from a finite-size
region, which naturally explains observed broken power-law spectra of the
middle-aged SNRs. In addition, the typical value of the break energy of the
gamma-ray spectrum, 1-10 GeV, is naturally explained from the fact that the
stellar wind dynamics shows the separation between the molecular clouds and the
explosion center of about 10 pc. We find that a runaway-CR spectrum of the four
middle-aged SNRs (W51C, W28, W44 and IC 443) interacting with molecular clouds
could be the same, even though it leads to different gamma-ray spectra. This
result is consistent with that of recent studies of the Galactic CR
propagation, and supports that SNRs are indeed the sources of Galactic CRs.
