Advanced ceramics are becoming widely used today for the mechanical components. However, it is quite difficult to machine these materials because of their high strength, hardness and brittleness. In order to increase the machining ability, the high pressure coolant is supplied to the cutting area from inside of the cutting tool through the tool holder and the spindle of machining center. For this mechanism, the side-through rotary union which transfer the coolant into rotating spindle is required, and so far the mechanical seal rotary unions have been applied. It is, however, considered to be difficult to apply the mechanical seal rotary unions under high speed rotation and high coolant pressure. In this study, a rotary union which use the hydrostatic pressure of coolant is proposed as one of methods to improve the performance of machine tools. The performances of developed rotary union are theoretically and experimentally investigated.