A hydraulic transmission comprising: a hydraulic pump of variable displacement type configured to be driven by mechanical energy input to a rotation shaft; and a hydraulic motor of variable displacement type configured to be driven by pressure oil from the hydraulic pump, wherein the hydraulic pump is configured such that displacement D P of the hydraulic pump is variable within a range from D Pmin to D Pmax, where D Pmin is a minimum value and D Pmax is a maximum value of the displacement D P, wherein the hydraulic motor is configured such that displacement D M of the hydraulic motor is variable within a range from D Mmin to D Mmax, where D Mmin is a minimum value and D Mmax is a maximum value of the displacement D M, wherein the minimum value D Mmin which is a lower limit of the variable range for the displacement of the hydraulic motor is equal to zero, wherein the hydraulic pump comprises: a plurality of pistons (12, 22); a plurality of cylinders (11, 21) configured to guide the plurality of pistons (12, 22) reciprocably and form a plurality of working chambers (13, 23) with the plurality of pistons (12, 22), respectively; at least one set of a first high pressure valve and a first low pressure valve provided for at least one first working chamber belonging to a first group among the plurality of working chambers; wherein at least one of the first high pressure valve or the first low pressure valve is constituted by a valve whose opening and closing is actively controllable so as to change an operation state of the at least one first working chamber between an active state where there is displacement volume and a non-active state where there is no displacement volume, the hydraulic pump further comprises at least one set of a second high pressure valve and a second low pressure valve provided for at least one second working chamber belonging to a second group among the plurality of working chambers, wherein the minimum value DPmin, which is a lower limit of the variable range for the displacement of the hydraulic pump is greater than zero, characterized by the second group being different from the first group, wherein each of the second high pressure valve and the second low pressure valve are constituted by a valve whose opening and closing is passively controllable so as to restrict the at least one second working chamber to an active state where there is displacement volume, and wherein the minimum value D Pmin of the variable range for the displacement of the hydraulic pump is determined in accordance with the number and volume of the second working chambers. a hydraulic pump of variable displacement type configured to be driven by mechanical energy input to a rotation shaft; and a hydraulic motor of variable displacement type configured to be driven by pressure oil from the hydraulic pump, wherein the hydraulic pump is configured such that displacement D P of the hydraulic pump is variable within a range from D Pmin to D Pmax, where D Pmin is a minimum value and D Pmax is a maximum value of the displacement D P, wherein the hydraulic motor is configured such that displacement D M of the hydraulic motor is variable within a range from D Mmin to D Mmax, where D Mmin is a minimum value and D Mmax is a maximum value of the displacement D M, wherein the minimum value D Mmin which is a lower limit of the variable range for the displacement of the hydraulic motor is equal to zero, wherein the hydraulic pump comprises: a plurality of pistons (12, 22); a plurality of cylinders (11, 21) configured to guide the plurality of pistons (12, 22) reciprocably and form a plurality of working chambers (13, 23) with the plurality of pistons (12, 22), respectively; at least one set of a first high pressure valve and a first low pressure valve provided for at least one first working chamber belonging to a first group among the plurality of working chambers; a plurality of pistons (12, 22); a plurality of cylinders (11, 21) configured to guide the plurality of pistons (12, 22) reciprocably and form a plurality of working chambers (13, 23) with the plurality of pistons (12, 22), respectively; at least one set of a first high pressure valve and a first low pressure valve provided for at least one first working chamber belonging to a first group among the plurality of working chambers; wherein at least one of the first high pressure valve or the first low pressure valve is constituted by a valve whose opening and closing is actively controllable so as to change an operation state of the at least one first working chamber between an active state where there is displacement volume and a non-active state where there is no displacement volume, the hydraulic pump further comprises at least one set of a second high pressure valve and a second low pressure valve provided for at least one second working chamber belonging to a second group among the plurality of working chambers, wherein the minimum value DPmin, which is a lower limit of the variable range for the displacement of the hydraulic pump is greater than zero, characterized by the second group being different from the first group, wherein each of the second high pressure valve and the second low pressure valve are constituted by a valve whose opening and closing is passively controllable so as to restrict the at least one second working chamber to an active state where there is displacement volume, and wherein the minimum value D Pmin of the variable range for the displacement of the hydraulic pump is determined in accordance with the number and volume of the second working chambers.