To provide a cooling device for a turbine capable of providing stability of an axial core by preventing increase of rotor length and weight, reducing cost, improving reliability, and making a turbine whole body compact, the turbine, and an assembly tool of the turbine.

A cooling structure is provided with: coolant channels 11, 12, 13 extending radial direction outward from an end part of a rotary shaft 2, formed as one unit with a rotary shaft 2, passing through an inside of a disk part 3 having moving blades attached on an outer circumference end thereof, and leading coolant to the moving blades; a storage part 14 constructing a part of the coolant channels 11, 12, 13 at the disk part 3, and storing an end part of a fastening part 5 fastening the disk part 3 and another disk part; a through-hole 24 formed at a wall part 22 opposing to the fastening part 5 at the storage part 14 and having a tool engaged with the fastening part 5 when the disk part 3 and another disk part are fastened; and a cover part 25 arranged on the through-hole 24 attachably and detachably.