A welding apparatus (1, 100) for welding together a plurality of overlaid sheet-like members by pressing in conjunction with application of vibration, comprising: a backup member (2, 102) including a backup surface (21, 121); a pressing body including a pressing surface configured to be smaller in the area than a welding range of the sheet-like members, and operative to press the sheet-like members against the backup surface (21, 121) in a pressing direction by means of the pressing surface; movement means (5, 58a, 58b, 59) for moving a portion of the sheet-like members that faces the pressing surface relative to the pressing surface in a direction substantially orthogonal to the pressing direction; and vibration generating means (4, 32) for vibrating the pressing body, wherein the vibration generating means (4, 32) comprises: an oscillator which is configured to apply ultrasonic vibration to the sheet-like members from the pressing surface by inducing the ultrasonic vibration in the pressing body; and a pressuring part for pressuring the sheet-like members by pressing the sheet-like members against the backup surface (21, 121) by means of the pressing surface of the ultrasonically vibrating pressing body, and wherein the pressuring part is configured to drive the pressing body, ultrasonically vibrated by the oscillator, to repeatedly pressure the sheet-like members during a predetermined pressuring time in one cycle of a preset low-frequency vibration period while maintaining the pressing body in low-frequency vibration with the low-frequency vibration period in a vibrational direction parallel to the pressing direction, characterized in that the movement means (5, 58a, 58b, 59) is configured to move the backup member (2, 102) in the direction substantially orthogonal to the pressing direction with a movement speed during the pressuring time set lower than a movement speed during a time period other than the pressuring time in one cycle of the low-frequency vibration period, the backup member (2, 102) being configured to carry the overlaid sheet-like members fixedly placed on the backup surface (21, 121) thereof. a backup member (2, 102) including a backup surface (21, 121); a pressing body including a pressing surface configured to be smaller in the area than a welding range of the sheet-like members, and operative to press the sheet-like members against the backup surface (21, 121) in a pressing direction by means of the pressing surface; movement means (5, 58a, 58b, 59) for moving a portion of the sheet-like members that faces the pressing surface relative to the pressing surface in a direction substantially orthogonal to the pressing direction; and vibration generating means (4, 32) for vibrating the pressing body, wherein the vibration generating means (4, 32) comprises: an oscillator which is configured to apply ultrasonic vibration to the sheet-like members from the pressing surface by inducing the ultrasonic vibration in the pressing body; and a pressuring part for pressuring the sheet-like members by pressing the sheet-like members against the backup surface (21, 121) by means of the pressing surface of the ultrasonically vibrating pressing body, and wherein an oscillator which is configured to apply ultrasonic vibration to the sheet-like members from the pressing surface by inducing the ultrasonic vibration in the pressing body; and a pressuring part for pressuring the sheet-like members by pressing the sheet-like members against the backup surface (21, 121) by means of the pressing surface of the ultrasonically vibrating pressing body, and wherein the pressuring part is configured to drive the pressing body, ultrasonically vibrated by the oscillator, to repeatedly pressure the sheet-like members during a predetermined pressuring time in one cycle of a preset low-frequency vibration period while maintaining the pressing body in low-frequency vibration with the low-frequency vibration period in a vibrational direction parallel to the pressing direction, characterized in that the movement means (5, 58a, 58b, 59) is configured to move the backup member (2, 102) in the direction substantially orthogonal to the pressing direction with a movement speed during the pressuring time set lower than a movement speed during a time period other than the pressuring time in one cycle of the low-frequency vibration period, the backup member (2, 102) being configured to carry the overlaid sheet-like members fixedly placed on the backup surface (21, 121) thereof.