PURPOSE: To automatically detect a dew deposition generating on the surface of a body in the atmosphere and making a cause of corrosion, by constituting the apparatus of sticking a resistance body and a conductive body having a different phase which is connected with equillibrated bridge circuit on the surface of the body in the state of insulating mutually.

CONSTITUTION: Several lead wires 1, 2 having a different phase and forming equillibrated circuit by a resistance body 5, a bridge circuit 6 and an amplifier 7, are arranged alternately and are stuck on the surface of measuring substance. When dew is condensed, resistance value of the body 5 becomes zero because of short- circuiting the wires 1, 2 of exposed part by sticking a drop of water and indication of a recorder 18 moves merely a varying quantity of the body 5 from a set up position. After that, that value is recorded continuously until the dew condensation disappears and when the dew condensation disappears, a short circuit is released and then, th indication of the recorder 8 is returned to the original set up position. Thus, the process and frequency from the production of the dew condensation to the disappearance is recorded by the recorder 8 continuously.