Automatic transmission

An automatic transmission comprising:

• a piston that has a first surface and a second surface opposing to each other in an axial direction and that is movable in the axial direction;
• a plurality of friction plates disposed on a side of the first surface of the piston;
• a fastening hydraulic chamber that applies hydraulic pressure to the second surface of the piston to move the piston to a fastening position where the friction plates are pressed to be fastened to each other;
• a release hydraulic chamber that applies hydraulic pressure to the first surface of the piston to move the piston to a release position where the friction plates are released;
• a hydraulic pressure control valve that has an output port of the hydraulic pressure of control valve to supply and discharge the hydraulic pressure to and from each of the fastening hydraulic chamber and the release hydraulic chamber;
• a first oil passage that allows the output port of the hydraulic pressure control valve to communicate with the fastening hydraulic chamber;
• a second oil passage that allows the output port to communicate with the release hydraulic chamber, and
• a predetermined-hydraulic-pressure change unit that changes the predetermined hydraulic pressure, wherein
• an area of the second surface of the piston for receiving the hydraulic pressure is set to be larger than an area of the first surface for receiving the hydraulic pressure,
• the second oil passage includes a reducing valve that prevents the hydraulic pressure of the release hydraulic chamber from exceeding a predetermined hydraulic pressure,
• and wherein
• the reducing valve includes a plurality of ports and a valve body that switches the plurality of ports,
• the plurality of ports include:
• a first port that communicates with the output port of the hydraulic pressure control valve,
• a second port that communicates with the release hydraulic chamber,
• a third port that discharges the hydraulic pressure of the release hydraulic chamber via the second port,
• a fourth port to which an output hydraulic pressure from the second port is applied and that moves the valve body in a first direction so as to reduce the output hydraulic pressure, and
• a fifth port that moves the valve body in a second direction opposite to the first direction,
• a regulation hydraulic pressure is applied to the fifth port, and
• the predetermined-hydraulic-pressure change unit changes the regulation hydraulic pressure so as to change the predetermined hydraulic pressure.