PURPOSE: To generate a simple and inexpensive waterproof protective film without incurring deterioration in the sensitivity by sticking a rubber elastic film with a specific thickness onto an ultrasonic reception face.

CONSTITUTION: In the manufacture of an underwater ultrasonic receiving element using a porous piezoelectric element, a rubber elastic film whose thickness is 10 to 100μm is stuck to an ultrasonic reception face. The thickness of the rubber elastic film is selected to be 10 to 100μm. When the thickness of the elastic film is less than 10μm, a flaw is caused easily by an external shock and somewhat fear is in existence in the water-proofing face and it is not preferred. On the other hand, when the thickness is over 100μm, the attenuation of ultrasonic wave vibration is increased and it is not preferred. Any type of material of the elastic film may be used so far as it has rubber elasticity at a room temperature (generally 0 to 40°C) and the thickness is in a specified range. For example, a silicon rubber or a polyisoplene is used. The elastic film is formed to have a prescribed thickness by sticking the film onto the reception face of the piezoelectric element.