A vehicular air conditioner comprising: a four-way valve which includes a piston mechanism in which a valve unit is shifted between one side piston room and the other side piston room, in a state where a discharge pressure of a compressor is acted to the one side piston room of the four-way valve, and an aspiration pressure of the compressor is acted to the other side piston room of the four-way valve, and the piston mechanism and the valve unit are shifted by using a differential pressure between the piston rooms, and the four-way valve is switched, whereby a cooling operation and a heating operation are switched; characterized in that a pressure accumulator (22; 23) is provided in which a coolant, which is related to the discharge pressure of the compressor, is accumulated when the compressor is operated, and the differential pressure, which is required for switching the four-way valve, between the piston rooms is maintained, and the differential pressure, which is required for switching the four-way valve, is generated between the piston rooms of the four-way valve even after the compressor is stopped, wherein the pressure accumulator is a high pressure accumulator of a coolant which is discharged from the compressor, and the high pressure accumulator is connected to a pipe, which passes the high pressure coolant which is discharged from the compressor, via a check valve in which a flow direction is a direction for flowing the coolant which is discharged from the compressor, and the high pressure accumulator is connected to the one side or the other side piston room via a control solenoid valve, whereby the high pressure coolant, which is discharged from the compressor, is supplied to the one side or the other side piston room. a four-way valve which includes a piston mechanism in which a valve unit is shifted between one side piston room and the other side piston room, in a state where a discharge pressure of a compressor is acted to the one side piston room of the four-way valve, and an aspiration pressure of the compressor is acted to the other side piston room of the four-way valve, and the piston mechanism and the valve unit are shifted by using a differential pressure between the piston rooms, and the four-way valve is switched, whereby a cooling operation and a heating operation are switched; characterized in that a pressure accumulator (22; 23) is provided in which a coolant, which is related to the discharge pressure of the compressor, is accumulated when the compressor is operated, and the differential pressure, which is required for switching the four-way valve, between the piston rooms is maintained, and the differential pressure, which is required for switching the four-way valve, is generated between the piston rooms of the four-way valve even after the compressor is stopped, wherein the pressure accumulator is a high pressure accumulator of a coolant which is discharged from the compressor, and the high pressure accumulator is connected to a pipe, which passes the high pressure coolant which is discharged from the compressor, via a check valve in which a flow direction is a direction for flowing the coolant which is discharged from the compressor, and the high pressure accumulator is connected to the one side or the other side piston room via a control solenoid valve, whereby the high pressure coolant, which is discharged from the compressor, is supplied to the one side or the other side piston room.