The prospect of developing a 750-mm gate valve for the JT-60 radio frequency heating system is discussed. In order to decrease the sealing force applied to the gasket, a sealing system with two H-type gaskets and intermediate vacuum chamber (double seal type design) is planned. Tests for 103 cycles repeated operation at room temperature and 150°C with a full-scale model of the valve seat confirm that valve seat leakage is less than 1.33×10−9 Pa·m3/s (1×10−8 Torr · ℓ/s) for helium gas and the sealing force applied to the gasket is less than 9.81×104 N/m (10 kgf/mm).