To operate at a high pressure according to an increase in external pressure during operation by low pressure fluid.

A part of a rod passage 12 is closed, and a passage 11 is closed during operation, and a cylinder 2, which is fitted with a piston 1a, which is integrated with a piston rod 14 having a pressure receiving area section 13 for initial operation, has first and second chambers 7 and 8, and a pipeline 5 for operation is communicated to a third chamber 10, which passes through the first chamber 7, passage 11 and rod passage 12 and is communicated to an external load. The second chamber 8 is kept charged with air, and when the supplied oil pressure is a given value or less, a cylinder device is operated at the supplied oil pressure, and when the supplied oil pressure is a given value or more, the cylinder device is operated at a high pressure caused by the projection of the piston rod 14 to the third chamber 10. By incorporating this device, a low pressure hydraulic equipment becomes a high pressure hydraulic equipment which outputs high pressure in a short time when required with ease at low cost. In addition, this device can perform independently with plural required number of passages branched from an operation pipeline and attaching this device to each of them respectively.