  We present our analysis of the magnetic field structures from 6000 au to 100
au scales in the Class 0 protostar B335 inferred from our JCMT POL-2
observations and the ALMA archival polarimetric data. To interpret the
observational results, we perform a series of (non-)ideal MHD simulations of
the collapse of a rotating non-turbulent dense core, whose initial conditions
are adopted to be the same as observed in B335, and generate synthetic
polarization maps. The comparison of our JCMT and simulation results suggests
that the magnetic field on a 6000 au scale in B335 is pinched and well aligned
with the bipolar outflow along the east-west direction. Among all our
simulations, the ALMA polarimetric results are best explained with weak
magnetic field models having an initial mass-to-flux ratio of 9.6. However, we
find that with the weak magnetic field, the rotational velocity on a 100 au
scale and the disk size in our simulations are larger than the observational
estimates by a factor of several. An independent comparison of our simulations
and the gas kinematics in B335 observed with the SMA and ALMA favors strong
magnetic field models with an initial mass-to-flux ratio smaller than 4.8. We
discuss two possibilities resulting in the different magnetic field strengths
inferred from the polarimetric and molecular-line observations, (1)
overestimated rotational-to-gravitational energy in B335 and (2) additional
contributions in the polarized intensity due to scattering on a 100 au scale.
