A motorcycle includes a motorcycle frame, a pivoting unit, a seat, and a motorcycle body cover which includes side covers. The base material metal of the extruded tube 2 in each of the joint portions 2a is subjected to a modifying treatment to produce finely divided crystal grains. A force-concentrating arrangement concentrates the opening force of the expanding airbag onto the tear seam, and comprises a structure extending around the airbag and having two opposite ends.