  We present polarimetric observations of the Keyhole Nebula in the Carina
Nebula Complex carried out using the Stratospheric Observatory for Infrared
Astronomy. The Keyhole Nebula located to the west of $\eta$ Carinae is believed
to be disturbed by the stellar winds from the star. We observed the Keyhole
Nebula at 89 $\mu$m wavelength with the HAWC+ instrument. The observations
cover the entire Keyhole Nebula spanning 8$'$ by 5$'$ with central position RA
= 10:44:43 and Dec = -59:38:04. The typical uncertainty of polarization
measurement is less than 0.5\% in the region with intensity above 5,500 MJy
sr$^{-1}$. The polarization has a mean of 2.4\% with a standard deviation of
1.6\% in the region above this intensity, similar to values in other high--mass
star--forming regions. The magnetic field orientation in the bar--shaped
structure is similar to the large--scale magnetic field orientation. On the
other hand, the magnetic field direction in the loop is not aligned with the
large--scale magnetic fields but has tight alignment with the loop itself.
Analysis of the magnetic field angles and the gas turbulence suggests that the
field strength is $\sim$70 $\mu$G in the loop. A simple comparison of the
magnetic field tension to the ram pressure of $\eta$ Carinae's stellar wind
suggests that the magnetic fields in the Keyhole Nebula are not strong enough
to maintain the current structure against the impact of the stellar wind, and
that the role of the magnetic field in resisting stellar feedback in the
Keyhole Nebula is limited.
