  We report a detailed study of the magnetic-field structure of the Crab pulsar
wind nebula, using the X-ray polarization data in 2--8~keV obtained with the
Imaging X-ray Polarimetry Explorer. Contamination of the pulsar emission to the
data of the nebula region was removed through application of a stringent
pulsation phase-cut, extracting a phase range of 0.7--1.0 only. We found that
the electric field vector polarization angle (PA) was about $130^{\circ}$ from
north to east with the polarization degree (PD) of about 25\% at the pulsar
position, indicating that the direction of the toroidal magnetic field is
perpendicular to the pulsar spin axis in the region close to the termination
shock. The PA gradually deviated from the angle as an increasing function of
the distance from the pulsar. There was a region of a low PD to the west of the
X-ray torus. Although such a region is expected to be located at the torus
edge, where geometrical depolarization due to a steep spatial variation of the
PA is expected, the observed low-PD region positionally deviated from the edge.
We found that the region of low PD positionally coincided with a dense filament
seen in the optical band, and conjecture that the low-PD region may be produced
through deflection of the pulsar wind. By comparing the values of the PD at the
pulsar position between the data and a model, in which toroidal and turbulent
magnetic fields were considered, we estimated the fractional energy of the
turbulent magnetic field to be about $2/3$ of the total. We also evaluated a
potential polarization of the northern jet in the nebula and derived the PD and
PA to be about $30\%$ and $120^{\circ}$, respectively.
