PURPOSE: To increase the rigidity, suppress the cost, and accomplish a lightweight construction.

CONSTITUTION: A straight main pipe 6 is coupled with a head pipe 24 through an approx. box-shaped coupling member 25. The main pipe coupling part of the coupling member 25 is formed wider than the front part in the direction across the width of motorcycle. The coupling member functions the same as the bend of a main pipe in conventional arrangement, and the load applied to the head pipe 24 is dispersed in the whole coupling member 25 and transmitted to the main pipe 6. Stresses are not likely to concentrate on the main pipe 6, and the rigidity is increased. Because of omission of any bend, the bending process can be omitted and the wall thickness be suppressed to necessary minimum.