  In order to investigate the far-infrared excess detected from the west hot
spot of the radio galaxy Pictor A with the Herschel observatory, a
submillimeter photometry is performed with the Atacama Compact Array (ACA) of
the Atacama Large Millimeter/submillimeter Array at Band 8 with the reference
frequency of 405 GHz. A submillimeter source is discovered at the radio peak of
the hot spot. Because the 405 GHz flux density of the source, $80.7\pm3.1$ mJy,
agrees with the extrapolation of the synchrotron radio spectrum, the
far-infrared excess is suggested to exhibit no major contribution at the ACA
band. In contrast, by subtracting the power-law spectrum tightly constrained by
the radio and ACA data, the significance of the excess in the Herschel band is
well confirmed. No diffuse submillimeter emission is detected within the ACA
field of view, and thus, the excess is ascribed to the west hot spot itself. In
comparison to the previous estimate based on the Herschel data, the relative
contribution of the far-infrared excess is reduced by a factor of $\sim 1.5$.
The spectrum of the excess below the far-infrared band is determined to be
harder than that of the diffusive shock acceleration. This strengthens the
previous interpretation that the excess originates via the magnetic turbulence
in the substructures within the hot spot. The ACA data are utilized to evaluate
the magnetic field strength of the excess and of diffuse radio structure
associated to the hot spot.
