  The pulsar B0458+46 was previously believed to have a distance of about
1.3$~$kpc and to be associated with a nearby supernova remnant, SNR HB9
(G160.9+2.6). We observe the neutral hydrogen (HI) absorption spectrum of PSR
B0458+46 by using the Five-hundred-meter Aperture Spherical radio Telescope
(FAST), and detect two absorption lines at radial velocities of $V_{\rm LSR} =
{-7.7}~{\rm km~s}^{-1}$ and $-28.1~{\rm km~s^{-1}}$. Based on the Galactic
rotation curve with a modification factor correcting for the systematic stream
in the anticenter region, we derive the kinematic distance of the farther
absorption cloud, which is found to be located $2.7^{+0.9}_{-0.8}$ kpc away,
just beyond the Perseus Arm. We also obtain a direct distance estimation of the
absorption clouds, being $2.3_{-0.7}^{+1.1}$ kpc, based on a comparison of
their velocity with the HI emission in the Perseus and Outer Arms that was
well-defined by recently measured parallax tracers. As a result, we conclude
that PSR B0458+46 should be located beyond the Perseus Arm, with a lower limit
distance of 2.7 kpc, and therefore not associated with SNR HB9. The doubled
distance indicates a deficiency of thermal electrons in the immediate outer
Galaxy, with much less density than current models predict. Additionally, we
detect a new high-velocity HI cloud in the direction of this pulsar.
