To provide a spindle supporting structure of a machine tool capable of heightening the rotational accuracy of a spindle by suppressing strain and thermal expansion of a quill and frame caused by heat generation of a bearing mechanism generated with rotation of the spindle and enhancing the machining accuracy of each work by a working tool.

A cylindrical quill 21 is fitted in a frame 20 and fixed, and a spindle 23 is supported in the quill 21 through bearings. A grinding wheel 27 is attached to the spindle 23. A space 35 to admit passage of cooling oil is formed between the peripheral surface of the quill 21 and the inner side face of the frame 20. To this space 35, the cooling oil is supplied externally so as to suppress heat generation caused by rotation of the spindle 23, and uneven thermal expansion of the frame 20 is suppressed.