A plating method of a rotating machine, which plates an inner surface of a casing of the rotating machine that has an opening for suctioning and discharging a fluid to an interior and exterior thereof, the plating method comprising: a surface activating process of supplying a pretreatment liquid into the casing through the opening, filling the casing with the pretreatment liquid, then discharging the pretreatment liquid from the casing through the opening, and activating the inner surface of the casing; a preheating process of supplying a preheating liquid into the casing through the opening, filling the casing with the preheating liquid, then discharging the preheating liquid from the casing through the opening, and preheating the casing after the surface activating process; and a plating process of performing supply and discharge of a plating liquid into and from the casing through the openings to circulate the plating liquid and plating the inner surface of the casing after the preheating process, the method wherein the plating work is performed by the circulation of the plating liquid by supplying the liquid into the casing from a lower side of the casing and discharging the liquid from an upper side of the casing, characterized in that when the vertical liquid level of the pretreatment liquid, the preheating liquid, and the plating liquid changes due to the supplying or discharging of the respective liquid during the surface activating process, the preheating process, and the plating process, respectively, the treatment liquid auxiliary supply device supplies each liquid to the inner surface of the casing which is located above the liquid level, and each of the pretreatment liquid, the preheating liquid, and the plating liquid is separately sprayed by the treatment liquid auxiliary supply device when the liquid is supplied and discharged from the openings of the casing. a surface activating process of supplying a pretreatment liquid into the casing through the opening, filling the casing with the pretreatment liquid, then discharging the pretreatment liquid from the casing through the opening, and activating the inner surface of the casing; a preheating process of supplying a preheating liquid into the casing through the opening, filling the casing with the preheating liquid, then discharging the preheating liquid from the casing through the opening, and preheating the casing after the surface activating process; and a plating process of performing supply and discharge of a plating liquid into and from the casing through the openings to circulate the plating liquid and plating the inner surface of the casing after the preheating process, the method wherein the plating work is performed by the circulation of the plating liquid by supplying the liquid into the casing from a lower side of the casing and discharging the liquid from an upper side of the casing, characterized in that when the vertical liquid level of the pretreatment liquid, the preheating liquid, and the plating liquid changes due to the supplying or discharging of the respective liquid during the surface activating process, the preheating process, and the plating process, respectively, the treatment liquid auxiliary supply device supplies each liquid to the inner surface of the casing which is located above the liquid level, and each of the pretreatment liquid, the preheating liquid, and the plating liquid is separately sprayed by the treatment liquid auxiliary supply device when the liquid is supplied and discharged from the openings of the casing.