A hydraulic transmission comprising: a hydraulic pump of fixed-displacement type configured to be driven by mechanical energy inputted into a rotation shaft; a hydraulic motor configured to be driven by pressurized oil from the hydraulic pump; a high-pressure oil line disposed between an outlet of the hydraulic pump and an inlet of the hydraulic motor, for guiding the pressurized oil from the hydraulic pump to the hydraulic motor; a low-pressure oil line disposed between an outlet of the hydraulic motor and an inlet of the hydraulic pump, for guiding working oil from the hydraulic motor to the hydraulic pump, the working oil having a pressure lower than a pressure of the pressurized oil; a bypass line connecting the high-pressure oil line and the low-pressure oil line; a bypass valve disposed in the bypass line; and a bypass-valve control part configured to control the bypass valve to adjust the pressure of the pressurized oil in the high-pressure oil line, wherein the hydraulic pump comprises a plurality of pistons, a plurality of cylinders configured to guide the respective pistons reciprocably and forming a plurality of working chambers with the plurality of pistons, and high pressure valves and low pressure valves provided for the plurality of working chambers, a check valve is used as the high-pressure valve and the low-pressure valve provided for each of the plurality of working chambers of the hydraulic pump, wherein the high pressure valves of the plurality of working chambers are check valves configured to permit only a flow of the working oil flowing from the working chambers to the high-pressure oil line, wherein the low pressure valves of the plurality of working chambers are check valves configured to permit only a flow of the working oil flowing from the low-pressure oil line to the working chambers, and wherein the bypass-valve control part is configured to, if a rotation speed (ωR) of the rotation shaft is not higher than a rated rotation speed, control the pressure of the pressurized oil in the high-pressure oil line to adjust a torque (τ) of the hydraulic pump to a target value on the basis of an optimum operation curve which defines a relationship between the rotation speed (ωR) of the rotation shaft and the torque (τ) of the hydraulic pump. a hydraulic pump of fixed-displacement type configured to be driven by mechanical energy inputted into a rotation shaft; a hydraulic motor configured to be driven by pressurized oil from the hydraulic pump; a high-pressure oil line disposed between an outlet of the hydraulic pump and an inlet of the hydraulic motor, for guiding the pressurized oil from the hydraulic pump to the hydraulic motor; a low-pressure oil line disposed between an outlet of the hydraulic motor and an inlet of the hydraulic pump, for guiding working oil from the hydraulic motor to the hydraulic pump, the working oil having a pressure lower than a pressure of the pressurized oil; a bypass line connecting the high-pressure oil line and the low-pressure oil line; a bypass valve disposed in the bypass line; and a bypass-valve control part configured to control the bypass valve to adjust the pressure of the pressurized oil in the high-pressure oil line, wherein the hydraulic pump comprises a plurality of pistons, a plurality of cylinders configured to guide the respective pistons reciprocably and forming a plurality of working chambers with the plurality of pistons, and high pressure valves and low pressure valves provided for the plurality of working chambers, a check valve is used as the high-pressure valve and the low-pressure valve provided for each of the plurality of working chambers of the hydraulic pump, wherein the high pressure valves of the plurality of working chambers are check valves configured to permit only a flow of the working oil flowing from the working chambers to the high-pressure oil line, wherein the low pressure valves of the plurality of working chambers are check valves configured to permit only a flow of the working oil flowing from the low-pressure oil line to the working chambers, and wherein the bypass-valve control part is configured to, if a rotation speed (ωR) of the rotation shaft is not higher than a rated rotation speed, control the pressure of the pressurized oil in the high-pressure oil line to adjust a torque (τ) of the hydraulic pump to a target value on the basis of an optimum operation curve which defines a relationship between the rotation speed (ωR) of the rotation shaft and the torque (τ) of the hydraulic pump.