To feed high-pressure oil of 70 mega Pascal to a load, by assembling a cylinder device having a pressure boosting mechanism with a pressure relief mechanism in a low-pressure hydraulic pump.

When low-pressure oil is fed to a load through a passage 13 and a secondary side oil chamber 11, a piston 1 is pressed by the operation of a valve mechanism 5 by the pressure increase, a pressure boosting piston rod 15 is moved to close the passage 13. In this condition, a portion 15a whose sectional area orthogonal to the axis is changed of the pressure boosting piston rod 15 is brought into contact with a seal packing 28 and leads to a parallel portion. The oil pressure generated during this operation is absorbed (taken in) by the movement of a piston rod 30 for pressure relief of a pressure relief mechanism 29.