A vehicle body component mounting system is provided with, along a conveyance path for a vehicle body, an upstream component mounting work area using a cell production system, a central component mounting work area in which the vehicle body is conveyed by an overhead conveyor, and a downstream component mounting work area using the cell production system in this order. Components other than principal components are mounted to the vehicle body using the cell production system. The cell production system is adaptable by increasing the number of component boxes, and thus an increase in the length of the line is prevented. The increase in the length of the line is prevented by using a line production system as a base and also using the cell production system as a part thereof.