A triaxial acceleration sensor which has a structure including a cover joined to a substrate including a mechanically operable functional unit to be sealed, is adapted in such a way that the joined state can be reliably obtained so as to not interfere with a displacement of the functional unit. A sealing frame is made of a heated polyimide on a periphery of an upper main surface of a substrate provided with a functional unit, and a sealing layer made of a polyimide is formed over an entire lower main surface of a cover. For integrating the substrate and the cover so as to seal the functional unit, the sealing frame and the sealing layer are joined to each other by heating and pressurizing the sealing frame and the sealing layer at a temperature that is about 50° C. to about 150° C. higher than a glass transition temperature of the polyimide while bringing the sealing frame and the sealing layer into contact with each other. In this case, a recess is formed in the vicinity of a portion of the sealing layer to be brought into contact with the sealing frame so that a bump, generated from the sealing layer which is deformed in the joining step, is prevented from protruding toward the functional unit.