A securing tool for a heat retention block covering a turbine casing main body includes: a securing rod, one end of which has an engaging portion with a protrusion; and a socket. The socket includes a guide groove, into which the protrusion of the securing rod is inserted, and a recessed groove. The guide groove includes a first guide groove, which extends in the socket axis direction from a starting end to a terminal end, and a second guide groove, the starting end of which is connected to the terminal end, and which extends from the starting end to a terminal end in a circumferential direction relative to the socket axis. The second guide groove is connected to the recessed groove.