27 a third small hole 30 1 and a fourth small hole 13 2 to be described below the first small hole 7 1 opens to the first plunger chamber 7 while the second small hole 27 opens to a portion of the compression chamber 3 near the discharge port 35 on the other hand the third small hole 30 1 opens to the inlet port of the solenoid valve 30 while the fourth small hole 13 2 opens to the second plunger chamber 13 1 fig7 and 8 are sectional views of the solenoid valve 30 in the closed state and opened state respectively the solenoid valve 30 has a construction known per se and includes a stationary iron core 30 8 a movable iron core 30 6 serving as a valve member a bobbin 30 10 surrounding the stationary and movable iron cores 30 8 and 30 6 and a coil 30 9 wound on the bobbin 30 10 a compression coil spring 30 7 is disposed between the stationary iron core 30 8 and the movable iron core 30 6 to urge both cores away from each other a seat member 30 11 is disposed on the side of the movable iron core 30 6 opposite to the stationary iron core 30 8 a first space 30 13 is defined between the movable iron core 30 6 and the seat member 30 11 and communicated with the third small.