  Recent high-cadence transient surveys and rapid follow-up observations
indicate that some massive stars may dynamically lose their own mass within
decades before supernovae (SNe). Such a mass-loss forms `confined'
circumstellar medium (CSM); a high density material distributed only within a
small radius ($\lesssim 10^{15}$ cm with the mass-loss rate of 0.01 $\sim
10^{-4} M_\odot$ yr$^{-1}$). While the SN shock should trigger particle
acceleration and magnetic field amplification in the `confined' CSM,
synchrotron emission may be masked in centimeter wavelengths due to free-free
absorption; the millimeter range can however be a potential new window. We
investigate the time evolution of synchrotron radiation from the system of a
red super giant surrounded by the `confined' CSM, relevant to typical type II-P
SNe. We show that synchrotron millimeter emission is generally detectable, and
the signal can be used as a sensitive tracer of the nature of the `confined'
CSM; it traces different CSM density parameter space than in the optical.
Furthermore, our simulations show that the `confined' CSM efficiently produces
secondary electrons and positrons through proton inelastic collisions, which
can become main contributors to the synchrotron emission in several ten days
since the SN. We predict that the synchrotron emission is detectable by ALMA,
and suggest that it will provide a robust evidence of the existence of the
`confined' CSM.
