POWER TRANSMISSION SYSTEM

A power transmission system is provided. The power transmission system includes a first transmission unit (28), a second transmission unit (24), a first engagement device (C1), a second engagement device (C2), a third engagement device (D1,C3), a fail-safe valve (86), and an electronic control unit (90). The electronic control unit (90) is configured to, during traveling in a state where the third engagement device (D1,C3) is released, output hydraulic pressure commands for simultaneous engagement of the first engagement device (C1) and the second engagement device (C2). The electronic control unit (90) is configured to, when it is determined that both the first engagement device (C1) and the second engagement device (C2) are engaged, prohibit traveling using the any one of the first power transmission path and the second power transmission path, which is connected or interrupted by the third engagement device.