PURPOSE: To provide an eccentric spindle, capable of changing the decentered quantity of the mounting hole of the tool, etc., of a main spindle in minutely and high accurately by relatively simple and inexpensive structure.

CONSTITUTION: First and second hilical gears 5 and 6, having reverse leads and the same number of tooth, are connected to a main spindle outer tube 2, pivottally supported on a device main body 1, and a main spindle 3, pivottally supported on the main spindle outer tube 2, and a movable mechanism part 7, having third and fourth helical gears 8 and 9, engaged with respective helical gears 5 and 6, is moved along an axis line direction by a first motor 12 to adjust decentered quantity. Consequently, an error, due to backlash and mechanical loss, is reduced, adjusting accurate decentered quantity.