To prevent an excessive adhesive from oozing in pre-bending, so that man-hours for hemming are reduced.

When forming a hemming part 6 for joining a 1st flange 10 of an upper panel 2 and a 2nd flange 12 formed at a tank cap 5, a bent end part 13 in the front end side of the 2nd flange 12 is bent on a bent part 19 at an acute angle from a flange base part 15, and an adhesive 17 is applied onto the flange base part 15. A front end part 11 of the 1st flange 10 is inserted into the acute angle part inside a bent part 14 while spreading the adhesive 17, and the bent end part 13 is bent with a pre-bending punch 25 having a front end of an inclined face 26. An excessive portion 18 of the adhesive 17 is accommodated in an adhesive pooling part 16 formed inside the bent end part 13, so that the oozing of the adhesive 17 is prevented.