A cooled platform for a gas turbine moving blade which enables the platform to be cooled when the moving blade is cooled by steam. The moving blade is provided with a plurality of steam passages. Steam is introduced from the steam passage at the trailing edge portion, flows in a serpentine passage composed of other steam passages to cool the blade, and flows out to a blade root portion from a base portion of the steam passage at the leading edge portion, being recovered. Part of steam flowing into the platform from the base portion of the steam passage at the trailing edge portion enters first and second steam passages in the platform. On one side, the steam passes through first, third, and fourth steam passages, and on the other hand, the steam passes through second, fifth, and sixth steam passages. The steam is recovered together with the steam having cooled the blade at the base portion of steam passage at the leading edge portion. Therefore, the peripheral portion of a platform can be cooled by steam, air is not needed, and the platform can be cooled by steam when the steam cooling system is used to cool the blade.