  We report Atacama Large Millimeter/submillimeter Array (ALMA) observations of
the binary system containing the pulsar PSR B1259-63 orbiting around a Be star
LS 2883 after the 2017 periastron passage. We detected radio continuum emission
from the binary system in the millimeter/submillimeter wavelengths for the
first time. At Band 3 (97 GHz), the flux 84 days after the periastron is almost
the same as that 71 days after the periastron. Although the binary system
showed intense GeV gamma-ray flares during our observations, the Band 3 flux
did not indicate any time correlation with them. The Band 3 fluxes are
consistent with an extrapolation of the radio spectrum at lower frequencies.
Assuming that it is synchrotron emission, we constrain magnetic fields
($\lesssim 0.6$ G) and the high-energy cutoff of the electrons ($\gamma \gtrsim
360$). The flux at Band 7 (343 GHz) 69 days after the periastron shows a
significant excess from the extrapolation of the radio spectrum at lower
frequencies. The flux may be associated with the circumstellar disk around the
Be star. We also present the results of Australian Telescope Compact Array
(ATCA) observations at 94 GHz for the 2014 periastron passage, which show that
the radio spectrum was relatively soft when the pulsar passed the disk.
