A method of maintaining a floating-body type wind turbine power generating apparatus including a wind turbine generator disposed on a floating body moored at a mooring position by a mooring line, the floating-body type wind turbine power generating apparatus being configured to supply electric power generated by the wind turbine generator to a cable, the method comprising: a separation step of separating the mooring line and the cable from the floating body of a first floating-body type wind turbine power generating apparatus including a maintenance-target section; a retention step of retaining the mooring line and the cable by a floating-body structure, after the separation step; a first transfer step of transferring the first floating-body type wind turbine power generating apparatus from the mooring position, after the separation step; a second transfer step of transferring a second floating-body type wind turbine power generating apparatus having no maintenance-target section to the mooring position; a connection step of detaching the mooring line and the cable from the floating-body structure and connecting the mooring line and the cable to the second floating-body type wind turbine power generating apparatus, after the second transfer step, and an operation-check step of checking operation of the second floating-body type wind turbine power generating apparatus at a site for operation check, before transferring the second floating-body type wind turbine power generating apparatus to the mooring position, wherein the floating-body type wind turbine power generating apparatus includes a ballast pump for floating and submerging the floating body, and a floating-body control center for controlling the ballast pump, characterised in that the ballast pump and the floating-body control center are disposed on a lower part of a tower of the wind turbine generator or the floating body, and wherein, in the operation-check step, electric power is supplied to the ballast pump from an electric power source under control of the floating-body control center, and the ballast pump is operated to check float/sink operation of the floating body in a state where only a lower part of a tower of the wind turbine generator is installed on the floating body, at the site for operation check. a separation step of separating the mooring line and the cable from the floating body of a first floating-body type wind turbine power generating apparatus including a maintenance-target section; a retention step of retaining the mooring line and the cable by a floating-body structure, after the separation step; a first transfer step of transferring the first floating-body type wind turbine power generating apparatus from the mooring position, after the separation step; a second transfer step of transferring a second floating-body type wind turbine power generating apparatus having no maintenance-target section to the mooring position; a connection step of detaching the mooring line and the cable from the floating-body structure and connecting the mooring line and the cable to the second floating-body type wind turbine power generating apparatus, after the second transfer step, and an operation-check step of checking operation of the second floating-body type wind turbine power generating apparatus at a site for operation check, before transferring the second floating-body type wind turbine power generating apparatus to the mooring position, wherein the floating-body type wind turbine power generating apparatus includes a ballast pump for floating and submerging the floating body, and a floating-body control center for controlling the ballast pump, characterised in that the ballast pump and the floating-body control center are disposed on a lower part of a tower of the wind turbine generator or the floating body, and wherein, in the operation-check step, electric power is supplied to the ballast pump from an electric power source under control of the floating-body control center, and the ballast pump is operated to check float/sink operation of the floating body in a state where only a lower part of a tower of the wind turbine generator is installed on the floating body, at the site for operation check.