A wind turbine generator or a tidal current generator comprising: a rotor; a generator which converts rotation energy transmitted from the rotor to electric power; and a hydraulic transmission which transmits the rotation energy of the rotor to the generator, wherein the hydraulic transmission includes: a hydraulic pump of variable displacement type which is driven by the rotor; a hydraulic motor of variable displacement type which is coupled to the generator; a high pressure oil line which is arranged between a discharge side of the hydraulic pump and an intake side of the hydraulic motor; and a low pressure oil line which is arranged between an intake side of the hydraulic pump and a discharge side of the hydraulic motor, wherein each of the hydraulic pump and the hydraulic motor of the hydraulic transmission comprises a plurality of working chambers each of which is surrounded by a cylinder and a piston moving slidingly in the cylinder, a cam configured to rotate with the shaft and having a cam surface which is in engagement with the piston, high pressure valves each of which opens and closes a high pressure communication path between each of the working chambers and the high pressure oil line, and low pressure valves each of which opens and closes a low pressure communication path between each of the working chambers and the low pressure oil line, wherein the hydraulic transmission is at least partially constituted of a plurality of modules, wherein the hydraulic pump comprises a cylinder block comprising a plurality of cylinders, and wherein the high pressure communication path of the hydraulic pump is arranged inside the cylinder block, characterized in that cylinder arrays each of which comprises a plurality of cylinders arranged in an axial direction of the rotation shaft include two adjacent cylinder arrays between which the high pressure communication path extends in the axial direction of the rotation shaft, and the high pressure communication path is in communication with the working chambers of the cylinders that belong to said two adjacent cylinder arrays on both sides of the high pressure communication path so as to extract pressurized oil from the working chambers to outside the hydraulic pump. a rotor; a generator which converts rotation energy transmitted from the rotor to electric power; and a hydraulic transmission which transmits the rotation energy of the rotor to the generator, wherein the hydraulic transmission includes: a hydraulic pump of variable displacement type which is driven by the rotor; a hydraulic motor of variable displacement type which is coupled to the generator; a high pressure oil line which is arranged between a discharge side of the hydraulic pump and an intake side of the hydraulic motor; and a low pressure oil line which is arranged between an intake side of the hydraulic pump and a discharge side of the hydraulic motor, wherein each of the hydraulic pump and the hydraulic motor of the hydraulic transmission comprises a plurality of working chambers each of which is surrounded by a cylinder and a piston moving slidingly in the cylinder, a cam configured to rotate with the shaft and having a cam surface which is in engagement with the piston, high pressure valves each of which opens and closes a high pressure communication path between each of the working chambers and the high pressure oil line, and low pressure valves each of which opens and closes a low pressure communication path between each of the working chambers and the low pressure oil line, wherein the hydraulic transmission is at least partially constituted of a plurality of modules, wherein the hydraulic pump comprises a cylinder block comprising a plurality of cylinders, and wherein the high pressure communication path of the hydraulic pump is arranged inside the cylinder block, characterized in that cylinder arrays each of which comprises a plurality of cylinders arranged in an axial direction of the rotation shaft include two adjacent cylinder arrays between which the high pressure communication path extends in the axial direction of the rotation shaft, and the high pressure communication path is in communication with the working chambers of the cylinders that belong to said two adjacent cylinder arrays on both sides of the high pressure communication path so as to extract pressurized oil from the working chambers to outside the hydraulic pump.