PURPOSE: To reliably seal a connection part, by a method wherein a tube is spread in diameter in a state that a press-in member made of resin, on which ultrasonic vibration is exerted, is pressed internally of a tube made of a metal to weld a tube to a tank and the press-in member.

CONSTITUTION: The tip part of a tube 4 is inserted in a communication hole 11 of a tank 2 or 3 along the direction of an arrow mark A. A press-in member 13 on which ultrasonic vibration is exerted is pressed in a hollow 12 of the tube 4 from the side facing the insertion direction of the tube. The press-in member 13 is gradually increased in an outer size in a range of from a tip 13a on the insertion side of the member 13 from a base end 13b, and the tube 4 is spread in diameter simultaneously with insertion of the member 13. The tank members 2 and 3, the press-in member 13, and the tube 4 are welded to each other in a way to melt resin of the contact surface by means of a friction heat occasioned by ultrasonic vibration. Since the whole of the inner and the outer peripheral surface of the tube 4 is welded, this method enables improvement of sealing ability of a connection part.