PURPOSE: To facilitate the making of a sensor to match with the outline of a transfer tube, by arranging a coil made up of a single wire formed in a solenoid on the circumference of the transfer tube to be measured to form a helical resonator.

CONSTITUTION: A single wire with the length equivalent to 1/4 wavelength of a water detection frequency signal is wound in a solenoid as single layer on the circumference of a tube body 1a made of fluorocarbon resin or the like with limited high frequency loss and with a wear resistance to form an internal conductor, which is opened at one end thereof while connected at the other end thereof to a box body serving as an external conductor 1c. The external conductor 1c is made of a better conductor such as aluminum, copper and brass and input terminals 1d and output terminals 1e which are arranged as opposed to each other sandwitching the internal conductor 1b are connected together securely at one end thereof respectively and high frequency connectors 1f and 1g are mounted as connected to the other end thereof respectively.