To provide a technique which can measure the thickness of an object to be detected such as a liquid film and so forth in real-time, and which can measure the velocity and the direction of a vapor-liquid interface.

The sensor 10 is composed of an approximately cylindrical casing 11, a shaft 12 disposed at the central part of the casing 11, two pairs of sensor parts 13A and 13B disposed between the casing 11 and the shaft 12, a filler 14 filled into the spacing among the casing 11, the shaft 12, and the sensor parts 13A and 13B. Each of the sensor parts 13A and 13B is composed of a pair of two electrodes 15a and 15b. By providing such a sensor 10 to a heat transfer tube 3 of a steam generator, the thickness of the liquid film L or the velocity of the vapor-liquid interface can be measured. Further, the flow of the vapor and the liquid in a group of the tubes of the vapor generator can be controlled by a controller 30.