It is an object of the present invention to provide a highly reliable image database system capable of accessing image data at high speed regardless of the increase of the image data and capable of controlling the image data as a whole. A storage unit 1, a control unit 2, an image database server 3 and a DICOM gateway 4 are subjected with each other to network connection. The storage unit 1 stores medical image data. The control unit 2 controls the storage unit. The image database server 3 stores key information associated with the image data stored and applies relay processing between an externally connected viewer and the image database server. The DICOM gateway 4 applies relay processing via a DICOM protocol between a plurality of modality devices externally connected with the image database server and the DICOM gateway. A control unit group, an image database server group, and a DICOM gateway group are constituted by providing at least a plurality of control units, a plurality of image database servers and a plurality of DICOM gateways, and a load balancer for executing load distribution control of every group is provided for every group based on header information of a request.