To provide a rotary member supporting unit capable of preventing grease from leaking outside by always maintaining its internal pressure at the approximately same pressure and stably rotating a rotary member at fixed rotary accuracy for a long period.

In the rotary member supporting unit U1, in which an internal pressure adjusting mechanism for maintaining its internal pressure at the approximately same pressure is provided, having a main spindle 8, a plurality of bearings 2, 4 for supporting the main spindle, at least one set of gears G1, G2 for transmitting rotary force of a drive shaft 38 to the main spindle, and the rotary member 32, an internal pressure adjusting chamber 20 for opening the inside of the internal pressure adjusting chamber to the rotary member supporting unit only and capable of collecting and accumulating a lubricant is arranged as the internal pressure adjusting mechanism, the internal pressure adjusting chamber is formed so as to expand and contract its capacity, and at least one connection hole for connecting with the inside of the rotary member supporting unit is provided. When the internal pressure in the unit rises, the capacity of the internal pressure adjusting chamber capacity is expanded by making air in the unit flow into the internal pressure adjusting chamber. When the internal pressure in the unit is reduced, the capacity of the internal pressure adjusting chamber is contracted by making the air in the internal pressure adjusting chamber flow into the unit. Consequently, the pressure in the unit is always adjusted at an approximately constant state.