  Fast radio bursts (FRBs) have excessive dispersion measures (DMs) and an
all-sky distribution, which point toward an extragalactic or even a
cosmological origin. We develop a method to extract the mean host galaxy DM
($\left\langle{\rm DM_{HG,loc}}\right\rangle$) and the characterized luminosity
($L$) of FRBs using the observed DM-Flux data, based on the assumption of a
narrow luminosity distribution. Applying Bayesian inference to the data of 21
FRBs, we derive a relatively large mean host DM, i.e. $\left\langle{\rm
DM_{HG,loc}}\right\rangle \sim 270~\rm{pc~cm^{-3}}$ with a large dispersion. A
relatively large ${\rm DM_{HG}}$ of FRBs is also supported by the millisecond
scattering times of some FRBs and the relatively small redshift $z=0.19273$ of
FRB 121102 (which gives ${\rm DM_{HG,loc}} \sim 210 ~\rm{pc~cm^{-3}}$). The
large host galaxy DM may be contributed by the ISM or a near-source plasma in
the host galaxy. If it is contributed by the ISM, the type of the FRB host
galaxies would not be Milky Way (MW)-like, consistent with the detected host of
FRB 121102. We also discuss the possibility of having a near-source supernova
remnant (SNR), pulsar wind nebula (PWN) or HII region that gives a significant
contribution to the observed ${\rm DM_{HG}}$.
