A metal collar according to the present invention, which is fitted to a through hole of an FRP material, comprises a first collar member and a second collar member. The first collar member is provided with an outer circumferential surface and an inner circumferential surface. The outer circumferential surface is in contact with the hole inner circumferential surface of the through hole. A silt is formed in one portion of the first collar member in the circumferential direction. The second collar member applies a pressing force to at least a part of the inner circumferential surface of the first collar member toward the radially outer direction, or alternatively applies a pressing force to inner surfaces of the slit toward the directions in which the inner surfaces get away from each other in the circumferential direction. The second collar member is held onto the first collar member by means of the reaction force of the pressing force, while pressing the outer circumferential surface of the first collar member against the hole inner circumferential surface by enlarging the diameter of the first collar member by means of the pressing force.