Various offshore structures such as long-span bridge, offshore wind farm, the undersea tunnel and man-made island, etc are now being constructed all around the world. Geotechnical investigations for offshore sites are necessary to the economical and efficient design of the structures. Offshore geotechnical investigations for a site with sea depth lower than 20m are usually accomplished by using SEP-barge and onshore equipment. However, it is very difficult to utilize SEP-barge for a site with sea depth higher than 30m because of severe circumstances including deep sea, high wave and tidal current etc. In this study, seabed type unmanned automated site investigation equipment, which can operate in deep sea conditions (sea depth : 100m, maximum boring depth : 50m), was newly introduced. The underwater equipment is designed to include the underwater boring module, common platform module, remote control module as well as SPT(Standard Penetration Test) module, which is widely utilized at onshore sites. This paper deals with the conceptual design of the equipment and specific mechanism of automated SPT system.