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.
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.