A securing method for a working system comprising: a floating type floating structure; and a working ship configured to perform at least installation or maintenance of the floating structure, wherein: the floating structure includes a column section whose peripheral surface is located at a waterline when the floating structure is floating, a ballast section arranged below the column section, and a flange section arranged at an intermediate portion of the column section, and the working ship includes a gripping section capable of engaging with an underside of the flange section, the floating structure includes a raising/lowering device configured to raise and lower the floating structure, and the securing method is characterized by comprising steps of: engaging the gripping section of the working ship with the underside of the flange section of the floating structure in a floating state, and lowering the floating structure, by the raising/lowering device, such that weight of the floating structure is borne by the gripping section via the flange section, to secure the floating structure to the working ship. a floating type floating structure; and a working ship configured to perform at least installation or maintenance of the floating structure, wherein: the floating structure includes a column section whose peripheral surface is located at a waterline when the floating structure is floating, a ballast section arranged below the column section, and a flange section arranged at an intermediate portion of the column section, and the working ship includes a gripping section capable of engaging with an underside of the flange section, the floating structure includes a raising/lowering device configured to raise and lower the floating structure, and the securing method is characterized by comprising steps of: engaging the gripping section of the working ship with the underside of the flange section of the floating structure in a floating state, and lowering the floating structure, by the raising/lowering device, such that weight of the floating structure is borne by the gripping section via the flange section, to secure the floating structure to the working ship. the floating structure includes a column section whose peripheral surface is located at a waterline when the floating structure is floating, a ballast section arranged below the column section, and a flange section arranged at an intermediate portion of the column section, and the working ship includes a gripping section capable of engaging with an underside of the flange section, the floating structure includes a raising/lowering device configured to raise and lower the floating structure, and the securing method is characterized by comprising steps of: engaging the gripping section of the working ship with the underside of the flange section of the floating structure in a floating state, and lowering the floating structure, by the raising/lowering device, such that weight of the floating structure is borne by the gripping section via the flange section, to secure the floating structure to the working ship. engaging the gripping section of the working ship with the underside of the flange section of the floating structure in a floating state, and lowering the floating structure, by the raising/lowering device, such that weight of the floating structure is borne by the gripping section via the flange section, to secure the floating structure to the working ship.