A wind turbine system comprising: a wind turbine generator in a wind power site of a plurality of wind turbine generators, said wind turbine generator comprising a tower provided to stand on land or off-shore, a nacelle supported on the tower to be controlled to rotate in the yaw direction and a plurality of rotatable blades attached to the nacelle to be controlled to move their pitch, and a portable terminal which includes a connecting portion connectable to connecting ends respectively provided on the lower portion of the tower and the nacelle so as to perform various control of the wind turbine generator, wherein the portable terminal further comprises an operation button for generating operation signals for various control devices that are incorporated into a control circuit in the wind turbine generator; a display portion configured to display the operating state of the operation button; and a control portion configured to perform display control of the operation button, wherein the control portion of the operation button is configured to make a mode selecting button to appear that selectively enables a maintenance mode or an operating mode of the wind turbine generator, and make an operation button to appear that separately controls at least one of: a blade pitch control, a yaw brake control and an accessory motor provided on an oil circuit to control the blade pitch control and the yaw brake control that are incorporated in the wind turbine generator, correspondingly to a corresponding maintenance operation screen displayed on the display portion when the maintenance mode is selected from the selecting button, wherein the control portion configured to perform display control of the operation button is configured to cause an operation button to appear or enables that operation button, only when the connecting portion is connected to the connecting end provided on the nacelle side, the operation button being operable only in the nacelle and configured to separately control the blade pitch control, the yaw brake control and the accessory motor. a wind turbine generator in a wind power site of a plurality of wind turbine generators, said wind turbine generator comprising a tower provided to stand on land or off-shore, a nacelle supported on the tower to be controlled to rotate in the yaw direction and a plurality of rotatable blades attached to the nacelle to be controlled to move their pitch, and a portable terminal which includes a connecting portion connectable to connecting ends respectively provided on the lower portion of the tower and the nacelle so as to perform various control of the wind turbine generator, wherein the portable terminal further comprises an operation button for generating operation signals for various control devices that are incorporated into a control circuit in the wind turbine generator; a display portion configured to display the operating state of the operation button; and a control portion configured to perform display control of the operation button, wherein the control portion of the operation button is configured to make a mode selecting button to appear that selectively enables a maintenance mode or an operating mode of the wind turbine generator, and make an operation button to appear that separately controls at least one of: a blade pitch control, a yaw brake control and an accessory motor provided on an oil circuit to control the blade pitch control and the yaw brake control that are incorporated in the wind turbine generator, correspondingly to a corresponding maintenance operation screen displayed on the display portion when the maintenance mode is selected from the selecting button,