A method for attaching a weather strip for a motor vehicle, which is capable of securely attaching a base portion of the weather strip to a predetermined position of a flange. The base portion including a bottom wall and an outer side wall and having a generally L-shaped cross-section is curved in a holding section of an attaching device on the side of an end of the flange to enlarge an angle between a bottom wall and an outer side wall of the base portion, and in a pressing section of the attaching device, the bottom wall is pressed against an end of the flange, and a double-sided adhesive tape bonded to the outer side wall is pressed against an exterior face of the flange, whereby the weather strip is attached to the flange.