  In this paper we investigate effects of neutral particles on shocks
propagating into the partially ionized medium. We find that for 120 km/s <
u_{sh} < 3000 km/s (u_{sh} is the shock velocity), about ten percent of
upstream neutral particles leak into the upstream region from the downstream
region. Moreover, we investigate how the leakage neutral particles affect the
upstream structure of the shock and particle accelerations. Using four fluid
approximations (upstream ions, upstream neutral particles, leakage neutral
particles and pickup ions), we provide analytical solutions of the precursor
structure due to leakage neutral particles. It is shown that the upstream flow
is decelerated in the precursor region and the shock compression ratio becomes
smaller than without leakage neutral particles, but the total compression ratio
does not change. Even if leakage of neutral particles is small (a few percents
of total upstream particles), this smaller compression ratio of the shock can
explain steep gamma-ray spectra from young supernova remnants. Furthermore,
leakage neutral particles could amplify the magnetic field and heat the
upstream region.
