A bearing unit capable of withstanding a large axial force based on a simple structure including: a bearing 10 having an internal ring 13 fitted to an inside member 2, an outer ring 14 fitted to an outside member 6, lots of rolling elements 15 disposed between the outer ring 14 and the inner ring 13; and a transmitting direction shifting means A transmitting an axial force F from the inner ring 13 to the outside member 6 when the axial force F transmitted from the inside member 2 to the internal ring 13 exceeds a predetermined value or above, and a main spindle device of a machine tool, which is provided with the bearing unit, are provided.
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