A lubricating structure of a rotational shaft oil sealing portion includes a casing, a rotational shaft rotatably retained by the casing, an annular space defined between the casing and the rotational shaft, an annular oil seal retained by the casing at one axial end of the annular space and sealing a clearance between the casing and the rotational shaft, the annular oil seal defining an oil seal chamber at the other axial end of the annular space, an oil supply passage, an oil drain passage, and an oil groove formed at the inner peripheral wall of the casing by cutting away the inner peripheral wall into a crescent shape in a circumferential direction, the oil supply passage communicating with the oil groove at a circumferentially center portion of the oil groove, wherein pressurized lubricating oil is supplied to the oil groove via the oil supply passage.