  We suggest and demonstrate in experiment that two normal metal /insulator/
superconductor (NIS) tunnel junctions combined in series to form a symmetric
SINIS structure can operate as an efficient Peltier refrigerator. Specifically,
it is shown that the SINIS structure with normal-state junction resistances 1.0
and 1.1 k$\Omega$ is capable of reaching a temperature of about 100 mK starting
from 300 mK. We estimate the corresponding cooling power to be 1.5 pW per total
junction area of 0.8 $\mu$m$^2$ at $T= 300$ mK.
