PURPOSE: To quickly detect abnormality of a circuit, in a pressure intensifying cylinder charging/discharging a hydraulic shaft coupling with a pressure oil.

CONSTITUTION: This is a pressure intensifying cylinder 301, 302 to supply pressure oil against a hydraulic shaft coupling which becomes in the engaged condition by enclosing supplied oil therein and in the disengaged condition by discharging the enclosed pressure oil, the rod 316 of the piston 314 of the pressure intensifying cylinder is projected outside a main body 311, and magnetic sensors S1-S4 as position detecting devices which convert the projected position of the rod 316 into a digital signal, are continuously provided. When the projected position of the piston rod 316 judged by a digital signal of combination of ON/OFF signals of the magnetic sensors S1-S4 is on the stroke end, it is judged that leakage is generated, when it is stopped on a prescribed position, the oil pressure is judged to be in the prescribed quantity, and when it is stopped this side of the prescribed position, it is judged that a blockade is generated.