A gas turbine rotating blade comprises a serpentine passage provided in the blade width direction in plural rows in a blade profile portion except the portion near the trailing edge having a small blade thickness, a steam cooling passage provided around the outer periphery of a platform, an air passage provided in the blade width direction in the vicinity of the trailing edge of blade profile portion, an impingement plate provided under the platform on the inside of the steam cooling passage, slot holes formed so as to branch off from the air passage and be directed to the trailing edge, and slits formed in the platform above the impingement plate. Therefore, the blade profile portion is cooled by the steam passing through the serpentine passage and the air passing through the air passage, so that the cooling construction is not complicated, and cooling is performed effectively. For the platform, the outer periphery thereof is cooled by steam and the inside thereof by air effectively.