A manufacturing method of a gas turbine blade (2, 4, 6), wherein the gas turbine blade (2, 4, 6) comprises: a hollow blade form portion formed by a leading edge on an upstream side of an working fluid of a gas turbine in a flow direction, a trailing edge on a downstream side of the working fluid in the flow direction, and a suction surface and a pressure surface reaching the trailing edge from the leading edge, and a shank portion for supporting the blade form portion, a partition (210a-210f) for connecting the suction surface and the pressure surface in a hollow region of the blade form portion, coolant paths (204a-204h) formed by the partition, the suction surface, and the pressure surface, an impingement cooling hole formed in the partition for dividing a first path which is a flow path nearest to the leading edge side among the coolant paths and a second path adjacent to the first path, a first converter portion which is installed between the blade form portion and the shank portion for connecting the second flow path and a third flow path adjacent to the second flow path, and a second converter portion which is installed at a tip on the blade form portion side for connecting the third flow path and a fourth flow path adjacent to the third flow path, wherein: at least, the first flow path and the second flow path are extended to an outer peripheral side end of the blade form portion, an outer peripheral side plate made of a different member from the blade form portion is attached on an outer peripheral side of the blade form portion and defines outermost peripheral ends of the first flow path and the second flow path and a gap formed between the partition for separating the first flow path and the second flow path and the outer peripheral side plate, wherein the manufacturing method of the gas turbine blade is comprising steps of: a process of manufacturing the blade form portion and the shank portion by precision casting using cores, a process of manufacturing the outer peripheral side plate as a different member from the blade form portion and the shank portion, a process of forming the gap at the outer peripheral side of the partition of the blade form portion for separating the first flow path and the second flow path so as to install a fluid interconnection portion other than the impingement cooling hole, and a process of attaching the outer peripheral side plate manufactured as a different member to the outer peripheral side of the blade form portion manufactured by casting. at least, the first flow path and the second flow path are extended to an outer peripheral side end of the blade form portion, an outer peripheral side plate made of a different member from the blade form portion is attached on an outer peripheral side of the blade form portion and defines outermost peripheral ends of the first flow path and the second flow path and a gap formed between the partition for separating the first flow path and the second flow path and the outer peripheral side plate, wherein the manufacturing method of the gas turbine blade is comprising steps of: a process of manufacturing the blade form portion and the shank portion by precision casting using cores, a process of manufacturing the outer peripheral side plate as a different member from the blade form portion and the shank portion, a process of forming the gap at the outer peripheral side of the partition of the blade form portion for separating the first flow path and the second flow path so as to install a fluid interconnection portion other than the impingement cooling hole, and a process of attaching the outer peripheral side plate manufactured as a different member to the outer peripheral side of the blade form portion manufactured by casting. a process of manufacturing the blade form portion and the shank portion by precision casting using cores, a process of manufacturing the outer peripheral side plate as a different member from the blade form portion and the shank portion, a process of forming the gap at the outer peripheral side of the partition of the blade form portion for separating the first flow path and the second flow path so as to install a fluid interconnection portion other than the impingement cooling hole, and a process of attaching the outer peripheral side plate manufactured as a different member to the outer peripheral side of the blade form portion manufactured by casting.