  A far-infrared counterpart to the west hot spot of the radio galaxy Pictor A
is discovered with the Spectral and Photometric Imaging REceiver (SPIRE)
onboard Herschel. The color-corrected flux density of the source is measured as
$70.0 \pm 9.9$ mJy at the wavelength of 350 $\mu$m. A close investigation into
its radio-to-optical spectrum indicates that the mid-infrared excess over the
radio synchrotron component, detected with WISE and Spitzer, significantly
contributes to the far-infrared band. Thanks to the SPIRE data, it is revealed
that the spectrum of the excess is described by a broken power-law model
subjected to a high-energy cutoff. By applying the radiative cooling break
under continuous energy injection ($\Delta \alpha = 0.5$), the broken power-law
model supports an idea that the excess originates in 10-pc scale substructures
within the hot spot. From the break frequency, $\nu_{\rm b} = 1.6_{-1.0}^{+3.0}
\times 10^{12}$ Hz, the magnetic field was estimated as $B\simeq1$-$4$ mG. This
is higher than the minimum-energy magnetic field of the substructures by a
factor of $3$--$10$. Even if the origin of the excess is larger than $\sim 100$
pc, the magnetic field stronger than the minimum-energy field is confirmed. It
is proposed that regions with a magnetic field locally boosted via plasma
turbulence are observed as the substructures. The derived energy index below
the break, $\alpha \sim 0.22$ (conservatively $<0.42$), is difficult to be
attributed to the strong-shock acceleration ($\alpha = 0.5$). Stochastic
acceleration and magnetic reconnection are considered as a plausible
alternative mechanism.
