1. A product development process support system comprising a processor which analyzes dependencies between a plurality of requirements belonging to any of a plurality of layers in product development processes, the system further comprising: an input reception portion receiving information representing the dependency between two requirements belonging to adjacent layers and of information representing the degree of influence of the two requirements on product development; and a dependency derivation portion deriving the dependency between two arbitrary selected requirements belonging to the same layer from information representing the dependencies between each of the two requirements and other requirements received by the input reception portion and from information representing the degree of influence of the two arbitrary selected requirements, based on dependency derivation rules established in advance, wherein the dependency derivation portion, (A) for all combinations of an arbitrary first requirement belonging to the first layer and an arbitrary second requirement belonging to the first layer, on all third requirement having values representing the dependency of the first requirement and the second requirement on a third requirement belonging to the second layer that are both equal to or above a predetermined strength, calculating (value representing the degree of influence of the first requirement)×(value representing the dependency of the first requirement on the third requirement)×(value representing the degree of influence of the third requirement)×(value representing the degree of influence of the second requirement), and maintaining the greatest value among the achieved values as the value representing the dependency of the first requirement on the second requirement in a DSM table format; (B) for all combinations between an arbitrary first requirement belonging to the first layer and an arbitrary third requirement belonging to the second layer, calculating (Value representing the dependency of the first requirement on the third requirement)×(value representing the degree of influence of the first requirement), and maintaining the achieved value as a value representing the dependency of the first requirement on the third requirement in a DSM table format; (C) for all combinations between an arbitrary third requirement belonging to the second layer and an arbitrary fourth requirement belonging to the second layer, on all first requirement belonging to the first layer, calculating (value representing the degree of influence of the third requirement)×(value representing the dependency of the first requirement on the third requirement)×(value representing the degree of influence of the first requirement)×(value representing the dependency of the first requirement on the fourth requirement), maintaining the greatest value among the achieved values as the value representing the dependency of the third requirement on the fourth requirement in a DSM table format; (D) for all combinations between an arbitrary third requirement belonging to the second layer and an arbitrary first requirement belonging to the first layer, calculating (value representing the degree of influence of the third requirement)×(value representing the dependency of the first requirement on the third requirement)×(value representing the degree of influence of the first requirement), and maintaining the achieved value as the value representing the dependency of the third requirement on the first requirement in a DSM table format.