Provided are: a working system for a floating structure, enabling the work of installation, maintenance, etc. of the floating structure to be performed in a stable state; a floating structure; a working ship; and a working method for a floating structure. A working system (1) for a floating structure is provided with a floating type floating structure (2), and also with a working ship (3) for performing at least the installation or maintenance of the floating structure (2). The floating structure (2) has: a column section (4), the peripheral surface of which is located at the water line when the floating structure (2) floats; a ballast section (5) disposed below the column section (4); and a flange section (6) disposed at the intermediate part of the column section (4). The working ship (3) has a gripping section (7) which can be engaged with the flange section (6), and also has a lifting/lowering means (8) for lifting and lowering the floating structure (2). The floating structure (2) is affixed to the working ship (3) by lifting and lowering the floating structure (2) and the working ship (3) while the gripping section (7) of the working ship (3) is engaged with the flange section (6) of the floating structure (2) which is in a floating state.