A pair of hydrostatic radial bearing devices 42, 43 is mounted on right and left side surface of a front portion of a wheel slide 34 in order to support rotatably wheel shafts 45, 52 respectively. A thrust bearing device 45 mounted in either one of wheel shafts 45, 52 supports either wheel shaft 45 or 52 in a thrust direction. A shaft coupling mechanism 60 is installed in wheel shafts 45, 52. A taper cylindrical portion 61 is projected from either one of wheel shafts 45, 52 and fitted tightly with a taper inside opening 65 formed in remaining of wheel shafts 45, 52. A vertical end surface 52t, 49t or Fb extending from a base of the taper cylindrical portion 61 is fitted tightly with another vertical end surface 45t, 52t or 45t of the remaining wheel shaft 45 or 52.