PURPOSE: To provide a steam amount sensor which is excellent in responseness and reproducibility and is suitable for a continuous control of humidity, by mounting foliated gold electrodes, whese one face contacts the air, and lead wires, connected to the electrode, on a piezo-electric body.

CONSTITUTION: Gold electrodes 2 and 2' are mounted to centers of the surface and the back of a crystal segment 1, acting as a piezo-electric body, so that they are positioned facing and opposite to each other, and a surface, making no contact with the crystal segment 1 of the gold electrodes 2 and 2', is exposed so that it makes direct contact with the air. Lead wires 3 and 3', consisting of a corrosion resisting metal such as stainless steel, rhodium, is led out from the gold electrodes 2 and 2', and are secured to a base 4 such as glass. A vibrating voltage is applied to the electrodes 2 and 2' to detect a resonance frequency, and this causes measuring of a steam amount in measuring gas with a precision and high-responseness. The device, consisting of a corrosion resisting meterial, is excellent in reproducibility and is suitable for continuous use.