  We present a precise thermometry system to monitor room-temperature
components of a telescope for radio-astronomy such as cosmic microwave
background (CMB) observation. The system realizes precision of 1 mK${\rm
\sqrt{s}}$ on a timescale of 20 seconds at 300 K. We achieved this high
precision by tracking only relative fluctuation and combining thermistors with
a low-noise measurement device. We show the required precision of temperature
monitors for CMB observation and introduce the performance of our thermometry
system. This precise room-temperature monitoring system enables us to reduce
the low-frequency noise in a wide range of radio-astronomical detector signals
observation and to operate a large detector array perform at its designed high
sensitivity.
