PURPOSE: To prevent the breakage of a hydraulic shaft coupling by sealing pressure oil supplied to the hydraulic shaft coupling with a check valve mechanism and by detecting the residual oil pressure when the pressure oil is exhausted with an open mechanism.

CONSTITUTION: Pressure oil supplied to a hydraulic shaft coupling is sealed with check valve mechanisms 37, 40. The check valve mechanisms 37, 40 are compulsorily opened with open mechanisms 46, 54 to exhaust the pressure oil. When the residual oil pressure is below a specified value, oil pressure applied to a piston 208 of a pressure detecting part 202 becomes lower than energizing force, and the piston 208 moves. Contact type or non-contact type sensors S1, S2 detect the move by a detecting piece 211 connected to the piston 208 and spindle shafts 1, 2 can be drawn out from the hydraulic shaft coupling. When the residual oil pressure exceeds a specified value, oil pressure to the piston 208 of the pressure detecting part 202 becomes higher than energizing force, and the detecting piece 211 does not move. Since the sensors S1, S2 does not detect any move, that is judged as abnormality, and drawing out of the spindle shafts 1, 2 is stopped to wait inspection.