In a power transmission shaft including a rear-side shaft having a male spline joint portion formed on the outer periphery of a front end of the rear-side shaft and a tubular front-side shaft having a female spline joint portion formed on the inner periphery of a rear end of the tubular front-side shaft, a seal ring is housed in a seal housing groove configured at the rear end of the tubular front-side shaft. The seal ring is interposed between an outer peripheral seal surface of the rear-side shaft and an inner peripheral surface of the seal housing groove with a compressive deformation. Additionally, the seal ring is sandwiched and retained between the inside wall surface of the seal housing groove and the inside end face of a disk-shaped retainer part of a retainer press-fitted to the rear end of the tubular front-side shaft.