SPINDLE UNIT

PROBLEM TO BE SOLVED: To provide a spindle unit provided with a pre-pressure changing mechanism capable of corresponding to a cutting condition of a wide range and having a compact constitution. SOLUTION: A main shaft 1 is accommodated in a easing 2 through a front side angular ball bearing 3 and a rear side angular ball bearing 7. An inner spacer 11 is inserted between inner races 4, 8 of the angular ball bearings 3, 7. A ring-like pad 21a, an outer spacer 12 and a ring-like pad 21b are axially inserted in series between outer races 6, 10 of the angular ball bearings 3, 7 in this order. The ringlike pads 21a, 21b are constituted by an inner diameter side member, an outer diameter side member, an edge surface member also using as an electrode, etc., and an electric viscous fluid is filled in a cavity formed in the inside thereof. A rigidity of the ringlike pads 21a, 21b can be varied by applying an electric field to the electric viscous fluid.