Since a portion of projected streaks on a friction- joining steel plate 10 is provided with a Vickers hardness which is at least 3 times greater than that on the surface of steel plates IA and IB, the projected streaks are allowed to more easily enter into the steel plates IA and IB, and since a frictional resistance derived from a mechanical slip resistance of the thus entered projected streaks acts between the steel plates IA and IB, a marked increase is obtained in the friction coefficient between the steel plates IA and IB via the friction-joining steel plate 10. Consequently, a friction-joining structure is constituted by a slip resistance mechanism which can be easily validated from a theoretical or an experimental point of view and is also small in variance of the friction coefficient. Therefore, a friction coefficient used in design can be set at a high value with high accuracy to realize a rational design.