A horn 39 and an anvil 49 are moved toward and away from each other by axially reciprocating a driving rod 29 and thereby reciprocatingly rotating a forked lever 31 in a predetermined angle range. The horn and the anvil are moved toward and pressed against each other with a predetermined force with a bag mouth b held therebetween. A sensor 61 detects the position of a second mounting block 53 secured to the rear end of a sliding shaft 47 of the anvil, thereby detecting the distance between respective pressing surfaces 40 and 50 of the horn and the anvil, i.e. the thickness m of a portion of the bag mouth held between the pressing surfaces 40 and 50. When the thickness m is not less than a predetermined threshold value M, a vibrator 35 is activated to perform sealing. When the thickness m is less than the threshold value M, no sealing is performed.