A plurality of indicated back-up pin devices are detachably placed on a back-up plate provided on a component mounting apparatus, to a plurality of indicated positions of the back-up plate to support a board from an under surface thereof, to which already mounted components have been mounted, when another components are to be mounted on an upper surface of the board. The back-up pin device comprises a base portion detachably placed on the back-up plate, at least one support pin erected upright on the base portion to support the board by contacting an upper end thereof with an under surface of the board and at least one of an identification mark for identifying the type of the back-up pin device and a position mark as a reference to determine a position of the base portion on the back-up plate, which is provided on the base portion.