A design management device for managing data has a unit design storage section for storing unit design data with corresponding part design data, a product design storage unit for storing product design data together with the corresponding unit design data, a data transmission unit for sending the product design data to the production management device together with the unit design data and the part design data, a transmission determination unit for determining whether or not the unit design data was transmitted in the past, and a transmission control unit that does not transmit corresponding part design data when the unit design data determined to have been transmitted in the past is sent.