A trunnion-type ball valve that can prevent fall-off of a ball seat from a seat retainer, can relieve the abnormal pressure rise in a cavity while avoiding abrasion of the ball seat, can improve workability and assembly performance by ensuring strength while suppressing increase in the number of parts and simplifying the structure, and can improve the durability of the ball seat by improving the sealing characteristics of the ball seat and a valve element even in a case of high pressure and preventing leakage is provided. A seat retainer 14 attaching a ball seat (s) 15 to one side or both sides of a ball 12 having a through hole 12a provided in a body 11 is disposed, the ball 12 is turnably provided via a stem 13, and the ball seat 15 is attached in a protrusion-prevented and free state in an attachment groove 32 formed in the seat retainer 14. An excessive pressure due to an abnormal pressure rise in a cavity 35 upon full close or upon full open is configured to be relieved into a flow channel via a communication part 40 provided between an inner peripheral surface 27 of the ball seat 15 and the attachment groove 32 by moving the seat retainer 14 in a direction opposite to the ball 12 by self-tension utilizing the pressure and pushing-out the ball seat 15 to a ball-12 side by the excessive pressure flowed into a rear-surface side of the ball seat 15 in the attachment groove 32.