PURPOSE: To protect an edge mode ceramic filter from the mechanical vibration or impact to increase the resistance to moisture, by immersing the ceramic filter in a low temperature molten material and then in the porous thermosetting resin to absorb the low temperature molten material.

CONSTITUTION: An element in which lead terminals 2", 3" and 4" are connected to an input terminal 2', an earth terminal 3' and an output terminal 4' respectively is immersed in the molten solution of a low temperature melting material like the paraffin or wax that is turned into a solid or a semisolid at the normal temperature and up to the position of about 1.5 times as much as piezoelectric ceramic plate 1 from the end face of the plate 1. The element is then covered with the 1st coated layer 7 having a thickness of 0.2∼0.4mm. Then the 2nd resin coated layer 8 is formed on the outside of the layer 7 by immersing the element including the terminals 2", 3" and 4" in the porous thermosetting resin and then heating and hardening it. Thus the low temperature molten material of the layer 7 is absorbed into the layer 8 to form a space near the layer 7.