To obtain a centralized monitoring device, a humidity detector and a moisture absorber to use for this device, which speedily detect the urine discharging state of a bedridden patient, etc., to inform a caring person.

A humidity detector (sensor) 12a is mounted within the diaper of a patient and a current of a prescribed oscillation frequency is supplied within a flexible antenna 8 from an oscillation circuit 23 through the antenna 8 arranged at a bed to generate the electric field of a prescribed frequency to inductively combine-synchronize with the sensor 12a. The output of the circuit is fluctuated by detuning when moisture is absorbed by the sensor 12a. This fluctuation is detected to inform a caring person.