An installation hole, in which a detector is accommodated, is formed in a substantially central portion of a body of a vortex flowmeter. A first ring is formed on an annular wall that forms a boundary between a first aperture portion and a second aperture portion of the installation hole. In the detector, a detecting element made up from a piezoelectric element is accommodated in a holder. A pair of second rings is formed on the outer circumferential surface of the holder. By inserting the detector in the installation hole, the second rings are placed in sliding contact with the inner circumferential surface of the installation hole, and the first ring comes into abutment with a boundary region of the holder. Consequently, a seal is formed between the detector and the body by the first and second rings.