A mounting structure for a mounting component, comprising: a mounting component that has a cylindrical portion, and a radially enlarged portion radially enlarged at one end of the cylindrical portion; and a first member that has a holding portion holding the mounting component and to which a second member is mounted by the mounting component, the holding portion being provided with: a housing space for the mounting component, the housing space being formed with a base portion, a side wall portion upstanding from the base portion, and a seat portion being continuous with the top of the side wall portion and having a bolt through hole into which a bolt is inserted; and a mounting component insertion hole that is provided in the side wall portion for inserting the mounting component into the housing space, wherein the bolt through hole is of such size as to abut on a peripheral surface of the cylindrical portion of the mounting component, and the holding portion is provided with: a communication portion that provides communication between the bolt through hole and the mounting component insertion hole, for allowing passage of the cylindrical portion of the mounting component into the bolt through hole; and restrictions that form a narrow portion in the communication portion, the narrow portion being narrower than an outer dimension of the cylindrical portion of the mounting component; characterised in that the seat portion is provided with lightening portions close to the restrictions. a mounting component that has a cylindrical portion, and a radially enlarged portion radially enlarged at one end of the cylindrical portion; and a first member that has a holding portion holding the mounting component and to which a second member is mounted by the mounting component, the holding portion being provided with: a housing space for the mounting component, the housing space being formed with a base portion, a side wall portion upstanding from the base portion, and a seat portion being continuous with the top of the side wall portion and having a bolt through hole into which a bolt is inserted; and a mounting component insertion hole that is provided in the side wall portion for inserting the mounting component into the housing space, the holding portion being provided with: a housing space for the mounting component, the housing space being formed with a base portion, a side wall portion upstanding from the base portion, and a seat portion being continuous with the top of the side wall portion and having a bolt through hole into which a bolt is inserted; and a mounting component insertion hole that is provided in the side wall portion for inserting the mounting component into the housing space, a housing space for the mounting component, the housing space being formed with a base portion, a side wall portion upstanding from the base portion, and a seat portion being continuous with the top of the side wall portion and having a bolt through hole into which a bolt is inserted; and a mounting component insertion hole that is provided in the side wall portion for inserting the mounting component into the housing space, wherein the bolt through hole is of such size as to abut on a peripheral surface of the cylindrical portion of the mounting component, and the holding portion is provided with: a communication portion that provides communication between the bolt through hole and the mounting component insertion hole, for allowing passage of the cylindrical portion of the mounting component into the bolt through hole; and restrictions that form a narrow portion in the communication portion, the narrow portion being narrower than an outer dimension of the cylindrical portion of the mounting component; the holding portion is provided with: a communication portion that provides communication between the bolt through hole and the mounting component insertion hole, for allowing passage of the cylindrical portion of the mounting component into the bolt through hole; and restrictions that form a narrow portion in the communication portion, the narrow portion being narrower than an outer dimension of the cylindrical portion of the mounting component; a communication portion that provides communication between the bolt through hole and the mounting component insertion hole, for allowing passage of the cylindrical portion of the mounting component into the bolt through hole; and restrictions that form a narrow portion in the communication portion, the narrow portion being narrower than an outer dimension of the cylindrical portion of the mounting component;