To estimate the water content on the real time basis with a simple constitution by rubbing a charge measuring member with an object to be measured, and measuring the quantity of the generated electrostatic charge.

When a microbial carrier 7 to be measured is rubbed with a charge measuring member 2, the electrostatic charge is generated and a charge 3 is accumulated in the charge measuring member 2. The charge 3 is connected to a capacitor 5 through an electrode 4 to accumulate the friction electrostatic charge in the capacitor 5. The charge quantity is calculated by measuring the voltage at both ends of the capacitor 5 by a voltmeter 6. The quantity of static electricity is changed according to the material of the microbial carrier 7 and the charge measuring member 2 and the change in water content in the microbial carrier 7. If the correlation between the water content in the microbial carrier 7 and the quantity of static electricity to be generated is obtained between the specified microbial carrier 7 and the charge measuring member 2 in advance, the water content in the microbial carrier 7 can be easily measured from the quantity of static electricity generated in the charge measuring member 2.