To provide a fluid pressure increasing/decreasing machine that can continuously supply an output pressure having a pressure higher than an input pressure and a pressure lower than the input pressure.

An oil pressure increasing/decreasing machine 100 capable of continuously supplying an output pressure includes: a control device 5 that switchably selects at least one input pressure chamber to which an input pressure is applied and at least one output pressure chamber in which an output pressure including a pressure higher than the input pressure and a pressure lower than the input pressure is generated, from a head side pressure chamber 1H and a rod side pressure chamber 1R of a hydraulic cylinder 1, and a head side pressure chamber 2H and a rod side pressure chamber 2R of a hydraulic cylinder 2; and flow control valves 6H, 6R, 7H, 7R that connect the input pressure chamber with a supply source SR, and connect the output pressure chamber with a supply destination SD.