A method for preventing a pressure in a crank chamber from excessively increasing without leading to an increase in weight or cost or causing damage to a pressurizing booster compressor. A compression chamber is formed by a cylinder, a cylinder head, and a piston reciprocal inside the cylinder. Rotation of an output shaft of an electric motor causes the piston to reciprocate via a crank shaft. A sealed crank chamber is defined inside a crank case. A pressurized gas supply tube supplies pressurized gas to be compressed into each of the compression chamber and the crank chamber. The crank case is formed from a hollow cylindrical body and an end plate facing the outside. A low-strength wall (a partition wall having a lower strength than another partition wall), which is configure to be broken under an allowable maximum pressure of the crank chamber, is formed on the end plate.