Variable discharge-amount compressor for refrigerant cycle

A compressor for a refrigerant cycle, driven by an engine for driving a vehicle, said compressor comprising:

• a compression unit having an inlet for sucking refrigerant, an outlet for discharging refrigerant and a compression room in which refrigerant is sucked and compressed, said compression unit being rotated with a rotation of the engine;
• and
• a discharge-amount varying unit which varies an amount of refrigerant discharged from said compression unit, said discharge-amount varying unit including
• a first control room communicating with said inlet and outlet of said compression unit,
• a control passage through which either one of said inlet and outlet of said compression unit communicates with said first control room,
• a valve for opening and closing said control passage,
• a second control room for applying a pressure to said valve in a first direction for closing said control passage,
• a third control room for applying a pressure to said valve in a second direction for opening said control passage, and
• an elastic member which applies an elastic force to said valve in the first direction, wherein:
• said discharge-amount varying unit reduces the discharge amount of refrigerant from said compression unit according to a pressure variation of said first control room;
• either one of said inlet and outlet of said compression unit communicates with either one of said second control room and said third control room;
• and
• said second and third control rooms communicates with each other to have a pressure difference therebetween, when a rotation speed of said compression unit is accelerated.