To provide a trunnion type ball valve which can reliably seal high-pressure fluid from a primary side and a secondary side according to a double seal method established by a repulsive force by the use of a spring and a fluid pressure, secure sealing property by generating a prescribed pressing force on the basis of the fluid pressure and adjust the pressing force to an optimum sealing surface pressure while securing the whole compactness and the strength of a seat retainer, suppressing the rise of a valve operation torque to enhance the durability and maintaining superior operability.

The trunnion type ball valve includes: a ball 12 disposed freely rotatably via a stem 11 in a body 14; and a seal mechanism 13 which is arranged on both side positions of the ball 12 such that the ball 12 comes into sealing contact with the seal mechanism 13. The seal mechanism 13 includes a ball sheet 25, a seat retainer 18, a spring member 22 and a seal member 20. The pressing force of the seal mechanism 13 is constituted with the repulsive force applied to the sheet retainer 18 by the spring member 22 and the fluid pressure set on the inner diameter side of the seal member 20.