To provide a sliding spline shaft device that can prevent leakage of lubricant oil even if a plug mounting part is torsionally deformed.

A plug 30 for closing an opening of a penetration hole 11a of a cylinder 11 on a side closer to a yoke 12 than to a female spline 11b includes: an annular resilient body 31 for radially outwardly pressing an inner peripheral surface of the penetration hole 11a; and a reinforcement member 32 secured on the annular resilient body 31 in a radially inward direction, and for regulating deformation of the annular resilient body 31 in the radially inward direction.