Provided are a power transmission shaft (shaft) which can achieve cost reduction by reducing a raw material diameter, and thus reducing a material cost and a lathing cost, and an assembly unit using the power transmission shaft. The power transmission shaft includes: a male spline (31) provided at an end portion of the power transmission shaft; an engagement large diameter portion (37) for preventing the power transmission shaft from being pushed in an axial direction of the power transmission shaft under a state in which the male spline (31) is fitted to a female spline (45) of another member; and a boot groove (33) for mounting a boot. When an outer dimension of the engagement large diameter portion (37) is denoted by D2, a maximum outer dimension of the male spline (45) is denoted by D1, and an outer dimension of a boot mount portion (32) provided with the boot groove (33) is denoted by D4, relationships D2D1 and D2=D4 are established. An outer diameter of the engagement large diameter portion (37) and an outer diameter of the boot mount portion (32) are set to be equal to the diameter of the raw material of the power transmission shaft to maintain the diameter of the raw material of the power transmission shaft.
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