To provide a seal device capable of improving seal performance between an inner circumferential surface of a through hole in a rod cover and an outer circumferential surface of a rod in a hydraulic device, and improving durability of a seal member.

This seal device seals gas in a gas chamber 15 in the through hole in a fluid pressure device 1 in which the rod 12 is movably inserted to the through hole in covers 13 and 14, with pressurized gas enclosed in or introduced to the gas chamber 15. The seal device 2 is composed of a circular oil chamber 21 in the inner circumferential surface 133 of the through hole, an oil passage 22 in the covers 13 and 14 to communicate outer surfaces of the covers 13 and 14 to the oil chamber 21, a pressure regulating passage 23 to communicate the oil chamber 21 with the gas chamber 15, and a pressure transmitting device 3 on the pressure regulating passage 23. The pressure transmitting device 3 has a transmitting member 32 disconnecting the pressure regulating passage 23, and movable or deformable in accordance with differential pressure between both sides. Oil is enclosed in the oil chamber 21, the oil passage 22, and the pressure regulating passage 23 on the communicating side to the oil chamber 21. Gas pressure in the gas chamber 15 is transmitted through the pressure transmitting device 3 to oil in the oil chamber 21.