  Radioactivity from radon is a major threat for high-precision low energy
physics experiments like the ones in the Kamioka Mine. We developed a new high
sensitive radon monitoring system and conducted systematic radon concentration
measurements for the first time in Kamioka. The system consists of portable
radon detectors with a capacity of 1 L and new electronics based on Raspberry
Pi. These radon detectors measure the radon in air with electro-static
collection and a PIN photodiode. We measured the absolute humidity dependence
of the 1-L radon detector for air to be $C_{F} (A_{H}) = (12.86 \pm 0.40) -
(1.66 \pm 0.19) \sqrt{A_{H}}$ $\mathrm{(counts/day)/(Bq/m^3)}$. The background
level of the 1-L radon detector is $0.65\pm0.15$ (stat.) count/day. This
corresponds to a detection limit of $\sim0.4$ Bq/m$^3$ in a one-day
measurement. Data was collected for a period of more than one year with twenty
one 1-L radon detectors in the Kamioka mine. They indicate seasonal and
day-night variations in radon concentration within the mine. These results also
allow us to confirm the stability of the new Raspberry Pi electronics.
